Understanding Transistor Circuits
August 1959 Popular Electronics
had been only a little over a decade since the transistor was invented
when this article appeared in the August 1959 edition of Popular Electronics.
Transistors were still a mystery to most people, including engineers,
technicians, and hobbyists. Author James Butterfield takes a unique
approach in presenting the material by writing it as a dialog between
an instructor and a student. If you are also new to transistors, this
will be worth your while to read.
August 1959 Popular Electronics
of Contents] People old and young enjoy waxing nostalgic about
and learning some of the history of early electronics. Popular Electronics
was published from October 1954 through April 1985. All copyrights are hereby
acknowledged. See all articles from
See all articles from
Understanding Transistor Circuits
A common-sense guide to using transistors
Canadian National Telegraphs
PART 1: Ohm's law and the Transistor
Pete introduces Johnny to the grounded emitter circuit. Its "why's"
and "how's" are investigated and the not-too-mysterious matters of base
and collector bias are discussed. The rule is, as Pete explains it,
"Let Ohm's Law be your guide."
PART 2: Design "Do's" and "Don'ts"
Trying to design a one-transistor amplifier, Johnny gets tangled up
with input and output impedance problems. Pete straightens him out with
some important "do's" and "don'ts" in getting a signal in and out of
3: Controlling, Coupling. and Testing
The transistorized approach
to filters and tone and volume controls is detailed by Pete with some
practical suggestions on the testing of transistor characteristics.
Coupling problems are discussed, and Pete draws up a chart that matches
the coupling capacitor to the collector resistor
Ohm's Law and the Transistor
should do it," grunted Pete, as he soldered a final connection. "Now,
let's tryout the amplifier."
"That's an amplifier?" snorted
Johnny. "You must have dreamed up that circuit during a fit of indigestion!
Anybody knows even the simplest transistor amplifiers are bristling
with resistors and oversize coupling capacitors. And where's the output
transformer for the speaker?"
"Don't need any." Pete clipped
on the ground lead from the audio generator and inserted a .1-µf. capacitor
in series with the other lead. "Trouble with most of you experimenters
is that you just copy diagrams - maybe change 'em a little here and
there to see what happens - but you never bother to find out what really
makes things tick." He touched the other end of the capacitor to his
input lead, and the speaker sang with a clear tone.
got six books on transistors, and I still don't know what I'm doing,"
complained Johnny. "They are either so simplified that they don't say
anything - or they're crammed with equations from stem to stern,"
"Nothing wrong with equations." Pete was busy tidying up his
workbench. "But you don't always need 'em. I threw this amplifier together
without fancy calculations - just horse sense. Let's go into the kitchen
for coffee, and I'll try to fill you in on the practical side of transistors."
Diodes Back to Back
off," said Pete settling in a chair, "You know that a transistor is
sort of like a pair of diodes, back to back, like this." He sketched
rapidly on a napkin.
"The way I've shown it here, the transistor
is a p-n-p unit. To get an n-p-n job, you flip both diodes end-for-end.
We'll dig deeper into this later-but for now, just remember when positive
voltage is on the p section and negative voltage on the n section, the
"I suppose the lead in the middle is the
base," said Johnny. "But which is the collector and which is the emitter?
Your diagram doesn't show any difference."
that will probably surprise you, Johnny. The collector and the emitter
could be changed around and the transistor would still work! But since
the manufacturer normally builds the collector bigger and stronger than
the emitter - take his advice and use the collector lead as the collector.
"The real difference between the collector and emitter is in
the way we treat them. The emitter is biased to allow maximum current
flow (forward bias) and the collector, on the other hand, is back-biased
for minimum current."
Johnny cut in. "Wouldn't a heavy current flow from the base through
the emitter mean that there would be darn little voltage drop between
the emitter and base?"
"Right, Johnny, there's seldom more than
a quarter of a volt difference between emitter and base, which makes
it easy to figure emitter current. Take this circuit for example:
"The collector isn't important just for the moment, so I won't
show any wiring to it. Since the emitter is at ground, you can figure
that the base is almost exactly at ground, too. This means ..."
"I get it," cried Johnny. "The battery puts exactly three volts
across the 1500-ohm resistor, so Ohm's law says that the current must
be ... ah ... two milliamps !"
chuckled Pete. "And since this part of the transistor is conducting
in the forward direction, the current will not be affected by the collector.
By the way, what type of transistor did I show in the diagram?"
"Negative voltage on the base, and positive on the emitter ... so
the transistor must be a p-n-p!" exclaimed Johnny triumphantly.
A Current Problem
point number two. The collector must be biased in the non-conducting
direction. How would you do that?"
"Put negative voltage on
it, I suppose," said Johnny thoughtfully. "But, wouldn't that mean that
no current would ever flow?"
would be true if the collector and base only were connected," replied
Pete. "But when the emitter is in the circuit with positive bias, an
interaction allows current to flow through the collector."
you're getting complicated," complained Johnny. "You sound like a textbook."
"Okay, let's put it this way. Whatever current flows through
the base will produce an amplified current through the collector. You
know how to find the base current - we just did it by Ohm's law. So
you take the base current (Ib), multiply it by your amplification
factor (Hfe) - and there's Ic, your collector
a minute," argued Johnny. "Suppose I put a higher voltage on the collector.
Wouldn't that make more current flow?"
"Not at all! Ignoring
the fine points, Ic will always equal Ib x Hfe.
The latest G.E. transistor manual, by the way, lists Hfe
for over four hundred transistors of all makes.
transistors, such as the 2N107 and the CK722, have gains of about 20.
Commercial grade transistors can run gains as high as 100 or more.
"Let's take a look at another circuit," Pete continued, sketching
rapidly. "See what you make of this."
me see," murmured Johnny. "With the emitter grounded, the base has to
be at almost zero volts - which will cause the battery's three volts
to drop across the 33,000-ohm resistor. That would give about 0.1 ma.
flowing through the base. If the amplification of the transistor is
20, that means the collector will take almost two milliamps. But, doesn't
the size of resistor R make any difference to the collector current
"Nope," said Pete cheerfully, "with one exception.
What would happen if that resistor were, say, 2000 ohms?"
... Ohm's law says that two milliamps times 2000 ohms makes a drop of
four volts. Oh, I see what you're getting at. If your collector resistor
is too big, it won't pass any voltage ... "
"And once you lose
the voltage, you lose your transistor action," finished Pete. "A resistance
of about 800 ohms would be dandy for this circuit."
give a collector voltage of - let's see - three volts minus a 1.6-volt
drop across the resistor leaves a little under a volt and a half for
the collector. Isn't that too low? You'd never be able to put out a
decent signal voltage with a supply like that."
what you're using the amplifier for. Usually, you don't need voltage
- all you want is current."
Beating the Heat
"Another thing, Pete, what about heat effects? Are transistors
as heat-sensitive as some of my books say? Does the gain change, or
"No, the gain of a transistor usually stays fairly
steady; but when the thermometer is high, a little leakage current sneaks
through. Don't worry about it, though. You'll seldom have trouble if
you stay well within the maximum ratings of the transistor."
"These 'max' ratings - anything complicated about them?"
"Gosh, no! Maximum collector-to-base voltage, maximum collector current,
and maximum collector dissipation - that's all you'll have to worry
about. They mean just what they say. Don't put too much voltage across
a transistor; don't put too much current through it; and don't heat
it up too much with a combination of the two.
"Now - you've
made a start on logical transistor design. The fine points will come
later. Whomp yourself up a few circuits. You'll find that theory and
practice go hand in hand. Then when you think you've got the basic idea
under your hat, we'll dig into this thing a little deeper."
Design "Do's" and "Don'ts"
I've been playing around with transistors like you told me, and I've
Pete carefully set down his soldering gun and
turned toward his visitor. "What seems to be the trouble, Johnny?"
"Well ... I wanted to boost the output of the radio I'm using
as a tuner. I laid out a circuit the way you showed me, but it doesn't
seem to work right. The volume is just as low as before - and what's
worse, the tone is mushy and distorted. I tried a new transistor with
no better results. Pete, tell me, where did I goof?"
"I think I know what the trouble is, Johnny. Your circuit is probably
good for a vacuum tube. You'll have to learn to think in different terms
for transistor amplifiers.
"First of all," said Pete, reaching
for a pencil and scratch pad, "do you remember how I told you that a
transistor was really a pair of diodes back to back?"
I got that part okay."
Now let's go back to fundamentals again and attack the problem from
a different position.
"Remember that the base-emitter 'diode'
must be hooked up to conduct in the forward direction. Let's look at
the circuit of an ordinary transistor amplifier.
"As I told
you last time, in a p-n-p transistor negative voltage is applied to
the base and positive voltage to the emitter, and conduction takes place
in the diode formed by the base and emitter. I'll sketch this part of
the circuit in detail for you.
"When a diode conducts, it's
just as if a switch has been closed. Now, when this base-to-emitter
diode is conducting, all your a.c. signal goes directly through the
diode to ground. For most purposes, the input impedance is so low that
you can consider it zero."
"Just a minute," interrupted Johnny.
"If the input is a short circuit, how can you get any amplification?
There won't be any voltage left at the input if you short it all out."
"Forget about voltages. Here's the important thing. Since the
transistor has a 'short-circuit' input, it may not have much voltage
at the input, but current flows into the transistor. Current is what
a transistor amplifies.
we have a transistor with a beta - that's current gain - of 20. Then
if I put one milliampere of signal into the base, I'll get 20 ma. at
the collector. Write this in your notebook in capital letters an inch
high - A TRANSISTOR AMPLIFIES CURRENT."
"Pete, I notice you
always put your signal into the base and take your amplified output
from the other collector. It seems to me I've heard of other ways of
connecting a transistor."
"Forget them, Johnny. There are very
few jobs that can't be handled by the grounded emitter. We'll talk about
the other circuits when an occasion arises that requires their use."
how do I go about using an output that's rated in milliamps instead
of volts? The current goes through the collector resistor, so it can't
be used as it stands."
"That's not exactly true, Johnny. It
depends on how you hook into the transistor output.
we put the output directly into another transistor stage," continued
Pete. "Since the input of this stage is a short circuit, all your signal
will be 'shorted' directly to this stage through the coupling capacitor.
In other words, all your output goes to the next stage.
both these transistors have a current amplification of 40, we can do
some fairly accurate figuring. An input of 0.5 ma. will produce 20-ma.
output. All of this goes to the second stage to be amplified, giving
you a final output of 800 ma. - almost a full ampere."
Johnny exclaimed. "That's a lot of current!"
"A little too much
current for comfort. To pass that amount of current, even in a power
transistor, you'd have to be careful of heat effects.
just mean mounting your transistors to dissipate heat - it means using
special circuits to compensate for heat effects.
"For the time
being, with the simple circuit we're using, it would be wise to keep
your output signal to a quarter of an ampere or less. At this level,
you can bring your d.c. bias down to about a third of an amp collector
current, which is well within safety limits for a low collector voltage.
If you use more than a couple of volts on the collector, keep an eye
on your power ratings. And use a heat sink, of course."
I still don't see how we're going to use that final output current.
Do you connect a coupling capacitor to the speaker or what?"
"You could do that, but you'd waste a lot of power. The easiest
way to do it is to put the speaker directly in series with the collector,
like this. But if the collector load is too small, there won't be enough
voltage drop across it for appreciable power to be transferred to the
speaker. Ohm's law, W=I2 R, (where W is the wattage delivered
to the speaker, I is the collector current, and R is the speaker impedance)
explains the problem.
"Get a transformer whose primary matches
the collector load requirements and whose secondary fits the speaker,
and you're in business."
"Let's get back to your original question on figuring your output.
If you're using a transistor as a preamplifier for a vacuum tube, you'll
have to know your voltage out. This is really very easy when you think
about it, because the input of the vacuum tube is usually high enough
this diagram, the total transistor load is the 5000-ohm collector resistor
shunted by a 1/2megohm volume control. For all practical purposes,
our output load is 5000 ohms. If our design shows we have a signal current
of one milliampere on the collector, we use Ohm's law ..."
tell me!" exclaimed Johnny. "That will be ... ah ... five volts!"
"You're sharp this morning. May I point out that if you want
to use a transistor in this way - and handle these levels - it would
be common sense to put at least seven volts or so on the collector?"
"I see what you mean, Pete. And I suppose it would also
be a good idea to have at least one milliampere of d.c. current through
"Right. Remember, of course, that a milliamp
of a.c. will draw almost 1.5-ma. peak current, so you should allow a
little extra. And that's just for pure sine waves - when you're figuring
in terms of average music levels, leave lots of room for swing.
"Generally, it's best to choose your d.c, bias currents keeping in mind
the a.c. signal voltages you want to handle," Pete went on. "And usually
it's best to have at least 1 ma. of collector current, even at low levels.
The reason for this is complex, having to do with impedances and distortion.
We'll save this topic for another time."
N-P-N or P-N-P
probably noticed that all the transistors I've shown have been p-n-p
types. As far as signal amplification goes, n-p-n transistors are exactly
the same. Your signal goes into the base and comes out from the collector.
Your connections for bias will be different, but that won't affect your
"If you use both types together, one emitter goes
to positive ground - that's the p-n-p - and the other emitter will go
to your negative voltage supply. This is quite okay, since they are
connected together signal-wise through the filter capacitor, making
both emitters 'grounded.'
"Your bias currents are easy
to set up on any of these stages. I've left the base and collector resistors
unmarked, for two reasons. First, the size of these resistors depends
on the battery voltage you are using. And, second, it will do you good
to work out the values yourself using the techniques I showed you before."
Getting the Signal In
discuss the booster you tried your hand on, now. Where did you connect
"Across the volume control. I wanted to be
able to turn the volume on the radio down and still feed my hi-fi."
"Well, let's sketch out a typical receiver circuit.
Notice that the detector is designed to work into a very high resistance
"I see !" exclaimed Johnny. "The transistor booster would
short out the volume control, and put a heavy load on the detector circuit.
Well ... how can I get the input impedance high enough so that it won't
affect the circuit?"
"That's a story in itself, Johnny. The
best way is usually to put a resistance in series with your input. But
even though your radio isn't an a.c./ d.c. job, you'll probably have
hum problems if you try using a common ground. For the moment I'll say
this: the best way will probably be to use an input transformer that
matches from 500K down to about 100 ohms.
"In the meantime,
dust off some of those old transistors and see if you can put them to
work. A lot of design 'failures' result from the experimenter forgetting
to take into account the very low impedance of transistors.
"Look me up when you've digested this session and I'll show you how
to check out some of the transistor design parameters, and incidentally
how to check out some of the transistor design parameters, and incidentally
how to test the little gadgets."
Coupling and Testing
It was a week later that
Johnny once again sauntered down the stairs into Pete's basement workshop.
Pete looked up from a small amplifier he was wiring. "How's tricks,
"I'm just a little confused, Pete. All this dope
you've been giving me on transistors is hard to digest so fast. For
example, you said that the input is a short circuit. Well, the Radio
Amateur's Handbook has a table of specs for quite a few transistors,
and for the 2N107 they say ... "
"They told you the input
impedance was around 600 ohms, I bet. Well, in a sense we're both correct.
Here, Johnny, look at it this way - you've done a fair amount of wiring.
How much resistance would you say there is in hookup wire?"
"I never thought about it. Oh ... maybe a half an ohm a foot ... but
it doesn't make much difference to the circuit."
"Because even if the resistance were as high as
an ohm or two, it's usually so much smaller than any other resistance
in the circuit that you don't count it. Who cares about half an ohm
or so when you're dealing with resistors of several thousand ohms?"
"Right you are, Johnny. And the same thing goes for your question
about a transistor input impedance. Even at six hundred ohms, it's so
much smaller than any other resistance in the circuit that you can count
it as a short."
Short and Simple
very simple to figure out a circuit this way, too. Using the short-circuit
input approach, you eliminate most of the calculating."
wouldn't that make your calculations inaccurate? You couldn't get the
exact values of gain. You'd have to be a few percent off."
at it this way, Johnny. To begin with, transistors are never rated exactly.
A transistor with a rated gain of 20 might have an actual gain as low
as 10 or as high as 40. So, fancy calculations usually aren't worth
Johnny nodded. "True. But let me draw you a circuit.
Now, if the voltage supply were very low - such as a single dry cell
- then the collector resistor R would have to be small enough so that
it didn't drop all the battery voltage. Wouldn't that mean ... ?"
Pete chuckled. "Johnny, you're too sharp for me today. Yes,
that would mean your small collector resistor would prevent some of
the signal from getting to the next stage. Not only would amplification
be poor, but the low resistance would call for some mighty hefty coupling
capacitors. And to put another fly in the ointment, as soon as your
signal began to get lost in the resistor, distortion would increase
sharply. But don't worry - unusually low voltages are almost the only
cause of such a situation."
"Pete, you lost me. I guess I still
don't dig this whole low impedance input idea."
brainwashed by vacuum-tube theory, Johnny - you've got to learn to rethink
the problems. Let's look at some volume controls."
vacuum-tube circuit volume control transferred to a transistor circuit
would look like this. Whereas a tube draws almost no grid current, the
base input of the transistor does - and right through the control. You
can see how moving the "pot" arm up and down is not only going to tap
off a part of the signal current, which is what we want it to do, but
unfortunately it's also going to change the value of the base bias resistor.
A volume control with no problems looks like this.
"If you run
into volume control problems such as distortion at low volume, abrupt
changes in volume as you move the control, or just excessive noise from
the control, check to make sure that you're not goofing up the bias
in some way and that there isn't too much current flowing through the
Capacitors - Filtering and Coupling
find that transistor filter circuits, too, have to be approached differently,"
Pete continued. "Here's a typical tube low-pass filter. A transistor
input following the filter would short out the capacitor, making it
"We'll redraw the circuit for transistor use like this.
It could function as a scratch filter or a simple equalizer for a magnetic
phonograph cartridge. The values, of course, depend on the use."
"Could I put two filter sections together, like this?" asked Johnny.
"You sure could. That will give you a sharper frequency cutoff."
"Looks like all you have to do to adapt a vacuum-tube circuit
for transistors is turn it end for end."
partly right, Johnny. But it applies only to coupling circuits between
transistors, and it won't always work. Remember to make the resistances
smaller and the capacitances larger than with tubes. And check out your
circuit practically after you work up the design."
of capacitors, Pete, I've been meaning to ask you: how do you figure
the size of your blocking capacitor?"
"Match it to your collector
resistor, usually. You know the formula - it goes, 'C equals over ...
" "Ouch! Spare me those formulas!" "Tarnation!" grumbled Pete. "If you
ever want to get anywhere in electronics, you'll have to start using
formulas. Things are getting too complicated for anyone to get along
with a wet finger and a screwdriver.
"This time I'll give you
a break. Here's a table for you. I've rounded off the values for a frequency
response down to about 20 cycles."
Johnny looked at his watch. "Pete, I'll have to run soon. But
first, you told me last time that you'd give me the low-down on how
to test transistors."
"Well, it's fairly easy. A nice thing
about transistors is that they usually don't gradually deteriorate-they
burn out. And you can spot almost all faults real quick by two simple
tests: leakage and gain.
makes the transistor easy 'to check - no fancy testers needed. Just
select n-p-n or p-n-p, check for leakage and current gain ... and that's
"Just how do you make these tests, Pete?"
take leakage. If I leave the base of a transistor disconnected, current
can't flow through the other two leads. The transistor is just two diodes,
back to back; so one of them will block the current flow.
this isn't quite true. A small leakage current will flow - usually less
than a tenth of a milliampere for most low and medium power transistors.
A reading over 1 ma. means your transistor has 'had it'.
to test for leakage, just place about six volts across the emitter and
collector, using the normal polarity for the transistor type. A milliammeter
with a range of anywhere from 1 - 10 ma. will do the trick. And for
the protection of the meter, it's a good idea to add a 2000-ohm series
resistor. This won't affect the accuracy of the test.
for gain is just as simple. Think back to our first talk. Whatever current
is put into the base of a transistor appears multiplied by the amplification
factor (beta) in the collector circuit."
exclaimed Johnny. "All you have to do is put a milliammeter in the base
circuit, another in the collector circuit - and you can see your amplification
can do it even easier than that," replied Pete. "With an exact known
current in the base, you need only one meter in the collector, which
you can calibrate to read directly in gain, if you wish. The ratio of
the collector current to the base bias current gives you your gain figure.
An open element, of course, gives you a zero - gain reading, as does
a base-to-emitter short.
"If a transistor's beta reads
low, don't throw it out. Remember, you're not working with vacuum tubes
where low gain usually means that the tube is wearing out.
"Power transistors are checked in the same way - but you'll find much
more leakage, sometimes as high as 10 ma. Because of this, it's easiest
to check this type at much higher current levels.
testers are quite inexpensive. Kit models are available, and it's a
lot easier to build yourself a finished tester than it is to rig up
a circuit every time you want to check a transistor.
another thing. You'd better learn how to use a little math! Those formulas
are there to help you. Dust off an old textbook, and ... "
"Yes, sir! Just as you say, sir! I'm on my way right now!" Johnny seized
the pile of circuit sketches and ran up the stairs. He waved back. "Thanks,
Pete. See you later !"
8-Watt Audio Amplifier
Driver Stage of the above amplifier is mocked up on this months
This class B audio amplifier will deliver 8 watts output
with less than .5 watt of audio drive. T1's impedance primary is chosen
to load the previous stage properly. Collector current in the first
stage is set at 70 ma. at a with R1. Pot R2 is adjusted for 100 ma.
at b. Both output transistors require the use of large heat sinks for
Code Practice Oscillator
transistor version of the Colpitts oscillator is used here as a code
practice oscillator. The 1- megohm tone control may be adjusted for
the most pleasing tone. Ordinary magnetic headphones are used and serve
as an inductance in the oscillator circuit.
Pocket Regen Receiver
detectors are easily adapted to transistor radio use. In this circuit,
coil L1 is a standard transistor antenna coil and the feedback winding
(L2) consists of 10 - 15 turns of wire wound around the center of L1.
The 1-megohm potentiometer is the regeneration control and should be
set just below the point where whistles or "motorboating" are heard
in the headphones. If no regeneration is heard, reverse the connections
to tickler feedback coil L2.
of the better broadcast-band receivers include an r.f. stage for increased
sensitivity. Extra r.f. amplification can be added to almost any receiver
without any complicated rewiring by means of a simple r.f. preamp such
as the one shown here.
above is a general-purpose crystal oscillator for use at frequencies
up to 20 mc. designed around a 2N94A transistor. Approximately 10 milliwatts
of r.f. output are developed at the crystal frequency. The low-impedance
pickup coil (L2) consists of two or three turns wound close to the lower
end of L1. Coil L1 and C1 are chosen to resonate at the crystal frequency.
is a code keyer unit designed to eliminate the key clicks and thumps
that may be cluttering up your transmissions. With the key up, the transistor
is cut off, as is the tube being keyed. With the key down, current drop
across the collector-emitter circuit is about 0.1 volt, and you're on
the air. Before adding the keyer unit to your transmitter, make these
checks. 1. Connect a 20,000-ohm voltmeter across your transmitter
key jack. If the voltage is greater than 25 volts, it may be necessary
to reduce the screen voltage on the keyed tube to 150 volts or less.
Use a voltage-regulated supply or voltage-divider network rather than
a simple series resistor. 2. Measure the current to be keyed by
placing a milliammeter across the key. This current should not be much
greater than 50 ma. 3. If it was necessary to lower the screen
voltage, check the r.f. output. If the output is too low, return the
screen voltage to normal and use two complementary transistors in series
back-to-back, as shown in the alternate circuit diagram. Note that the
keyer should never be left in the key-down condition.
Dynamic Microphone Preamp
high-grade dynamic microphones have too low an output to be used with
tape recorders or p.a. systems. A simple self-contained preamp can be
designed using one transistor which will boost the output of these low-level
microphones sufficiently for almost any type of application.
miniature hand-held wireless microphone can be constructed using an
r.f. transistor in a Hartley-type oscillator. Good short-range results
can be obtained on the broadcast band with a 2' whip antenna. Follow
the manufacturer's coil terminal numbering shown or oscillation will
not occur. Adjust the coil and trimmer for best reception on a clear
spot in the broadcast band.
Sensitive Photo Relay
two-stage direct-coupled circuit offers high sensitivity in photocell
applications. A 1-megohm pot is included for sensitivity adjustment
over a wide range. The relay is a sensitive Advance Type SO/1C/4000D
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