# Chapter 7: AC Transistor Amplifiers

```Chapter 7: Transistors, part 2
Chapter 7: AC Transistor Amplifiers
The transistor amplifiers that we studied in the last chapter have some serious
problems for use in AC signals. Their most serious shortcoming is that there is a “dead
region” where small signals do not turn on the transistor. So, if your signal is smaller than
0.6 V, or if it is negative, the transistor does not conduct and the amplifier does not work.
Design goals for an AC amplifier
Before moving on to making a better AC amplifier, let’s define some useful terms.
We define the output range to be the range of possible output voltages. We refer to the
maximum and minimum output voltages as the rail voltages and the output swing is the
difference between the rail voltages. The input range is the range of input voltages that
produce outputs which are not at either rail voltage.
Our goal in designing an AC amplifier is to get an input range and output range which
is symmetric around zero and ensure that there is not a dead region. To do this we need
make sure that the transistor is in conduction for all of our input range. How does this
work? We do it by adding an offset voltage to the input to make sure the voltage
presented to the transistor’s base with no input signal, the resting or quiescent voltage, is
well above ground. In lab 6, the function generator provided the offset, in this chapter we
will show how to design an amplifier which provides its own offset.
Now that you understand capacitors it is pretty easy to see how to add and subtract an
offset voltage to a signal, at least for AC signals. From here on, you will design transistor
circuits with a bias network. This bias network is simply a voltage divider that is
connected to the input. Its job is to insure that the output stays at approximately half the
supply voltage for small input signals. Then, the output voltage can vary over a wide
range (positive and negative) while always keeping the transistor conducting.
The trick to make this work is to separate these quiescent voltages and currents from
the input and output signals. To do this, you will use blocking capacitors to isolate the
input and the output. If you connect an AC input to a capacitor, it will not pass any DC
offset voltages but it does pass the fast AC signals. Similarly, an output blocking
capacitor will pass the fast signal while keeping the quiescent (resting) voltage from the
amplifier from disturbing whatever comes next. This is shown schematically in the first
figure (next page).
Some Design Basics
This week we are going to redesign our emitter follower and inverting amplifier to
use bias networks. To help you with your design, we will make a step by step list for
designing each of these basic transistor circuits.
Here are a couple initial design decisions we will make
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Chapter 7: Transistors, part 2
•
You will begin by determining the quiescent (DC, no signal) current through the
collector. You usually want the quiescent current to be larger than any current you
will use to drive a load. A quiescent current of 1 mA is typical, and we will use
that in our example designs.
•
We will also use a single +15 V power supply to power the collector (the common
collector voltage or VCC) for and the bias network.
•
transistor’s base current will load the output of our bias voltage divider. To bias
the base, we need a stiff voltage divider (i.e. low impedance). Our rule of thumb
for designing voltage dividers was to have a factor of 10 difference in impedance
at each stage.
AC Emitter-Follower
Design steps for the emitter-follower of figure 7.1 proceed as follows:
1. To have the maximum symmetric range of output voltages we would like our
quiescent base voltage to be half of the (15 V) supply voltage. So, we will use a 1:1
input voltage divider. This means that both biasing resistors will be the same.
2. We then choose the emitter resistor. The
quiescent voltage at the emitter is a diode
drop below the voltage in the middle of
the bias network (i.e. Vcc/2 if we have a
1:1 divider). This is roughly +7 V in our
case. To get our design quiescent current,
the emitter resistor must be
Re = Ve/Ie = 7 V / 1 mA = 7 kΩ.
VC
CIN
R1
VIN
We will use the standard 6.8 KΩ. It is
close enough.
COUT
R2
VOUT
3. To bias the base we want a stiff voltage
Re
divider (i.e. low impedance), therefore we
want to use resistors that are smaller than
the
base-emitter-ground
impedance
(Zb=Vb/Ib ~ 750 kΩ) by a factor of 10 or
so. For this example, we will choose two
75 kΩ resistors for this divider.
Figure 7.1: A biased emitter-follower
4. Remember to AC couple (via npn transistor amplifier.
capacitors) the input and output. The exact
values are not particularly important, though you should remember that you are
making a biased high-pass RC filter. Values around 0.1µF are typical if you want
f3dB~ 20 Hz, but you can use what you have as long as it is not too small.
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Chapter 7: Transistors, part 2
Common-Emitter (Inverting) Amplifier
In this circuit, we need to know the quiescent current and the desired gain. Let’s assume a
gain of -5 and a 1mA quiescent current for this example. The circuit diagram is shown in
figure 7.2.
1. In this circuit we want the quiescent
output (at the collector) to be set roughly
halfway between the power supply and
the ground for maximum output voltage
swing. For Ic = 1 mA,
Vcc
Rc = Vc/Ic ≈ 7 V / 1 mA = 7 kΩ.
We will use a standard resistor of about
this value (e.g. Rc = 6.8 KΩ) as we did
for the follower.
2. The emitter resistor can be determined by
the desired gain. Previously we saw that
C1
R1
Rc
C2
VOUT
VIN
R2
Re
Gain = -Rc /Re
In our case we want a gain of 5 so we
chose Re = 1.35 kΩ. We will approximate
this by a standard 1.5 kΩ resistor. Note
that with this choice, the emitter
quiescent voltage will be given by the
voltage drop across the emitter resistor
Figure 7.2: A biased npn transistor
inverting amplifier.
Ic Re = 1.5 V.
3. The tricky part is to design the bias network for this circuit. Since we know the
emitter voltage, the output of the bias network (i.e. the base voltage) is just a
diode drop higher than the emitter voltage. Therefore the bias resistors must be set
to give a base voltage, Vb, of
Vb = V2 = Ve + 0.6 V
= 1.5 V + 0.6 V
= 2.1 V.
This is the drop across the lower of the two bias resistors. The other has a drop of
V1 = Vcc – V2
= 15 V – 2.1 V
= 12.9 V
The bias network’s output impedance must be small enough to keep this bias up
even when loaded. We will select the bias resistors so that the current running
through the bias network is about 10 times larger than the current that goes into
the base. If β=100, then the base current is
Ib = Ic/β = 10 µA,
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Chapter 7: Transistors, part 2
and the bias current, Ibias, is ten times larger: Ibias = 100 µA. The total resistance of
the bias network is R1+R2, so we need
R1+R2 = (15 V) / (100 µA) = 150 kΩ.
We can now determine the value for R2, since from the voltage divider formula,
we require:
Vb / Vcc = R2 / (R1+R2) ⇒ R2 = (2.1 V / 15 V) ⋅ (150 kΩ) = 21 kΩ,
and therefore,
R1 = 150 kΩ - 21 kΩ = 129 kΩ.
4. Remember to AC couple the input and output with capacitors. The values are not
particularly important, but the relevant RC time constants should be chosen so as
to guarantee unimpeded passage of the AC signal to be amplified.
Design Exercises
Design Exercise 7-1: Design an AC emitter follower with a 1.5 mA quiescent current.
Remember that you can approximate the resistor values by picking the nearest standard
values. Please show any approximations when they are employed.
Design Exercise 7-2: Design an AC inverting amplifier which should operate at
frequencies above 100 Hz with a 0.2 mA quiescent current and a gain of 15.
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Chapter 7: Transistors, part 2
Lab 7: Transistor Applications
1. Transistor Switch (30 minutes … 1 hour)
Construct a voltage controlled transistor switch by connecting the collector of a
2N3904 transistor though a light bulb to a 6 V power supply (see design exercise 6-2).
Use a square wave voltage to control the switch. Measure the voltage across the light
bulb and compare with your quantitative and qualitative results from lab exercise 4-1 (if
necessary repeat lab exercise 4-1).
2. DC-biased AC transistor amplifier (2 hours … much longer if not prepared)
a. Design and construct a common-emitter amplifier with a quiescent current I C =
0.2 mA, and a gain of ~15 at 2 kHz, powered by a +15 V power supply. Measure the
small-signal AC gain, and compare it to your calculation. Measure the voltage swing
(i.e. the maximum output voltage swing before distortion). Measure the output
impedance (Suggestion: use AC coupling).
b. Short your emitter resistor to obtain the
maximum gain. Measure the new small-signal
gain. It will be necessary to change your bias
network. Does this agree with your expectations?
Measure the new voltage swing. Measure and
describe any distortion.
a gain of 15. Now try to eliminate the emitter
resistor without affecting the DC bias, by using a
by-pass capacitor parallel to the emitter resistor.
Measure the new small-signal gain. Measure and
describe any distortion.
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+15V
RC
R1
VOUT
VIN
R2
RE
Chapter 7: Transistors, part 2
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