Project 19 Push-Pull (Class AB, B) Amplifiers

amountdollElectronics - Devices

Nov 2, 2013 (3 years and 5 months ago)

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Project 19

Push
-
Pull (Class AB, B) Amplifiers

Objective:

This project will investigate the operation of a push
-
pull amplifier. Class B and Class AB
operation will be examined.

Components:
2N2222 BJT, 2N2907

Introduction:

Many small signal amplifiers, both

BJT and FET, are biased to provide symmetric output voltage swings.
This bias arrangement, as examined in many of the previous projects, requires a DC collector (drain) current
and a DC collector
-
emitter (drain
-
source) voltage even when no input signal is

being amplified. The product
of this bias current and voltage results in the transistor dissipating power at all times. The class of amplifiers
that have a continuous power dissipation are referred to as
Class A

amplifiers. The Push
-
Pull amplifier
illust
rated in Figure 19
-
1 is referred to as a
Class B

amplifier since the transistors are off unless an input
signal is present and there is one transistor used for positive and one transistor used for negative output
voltage swings. In this arrangement, there
is no DC power dissipated by the transistor. The transistors
illustrated in Figure 19
-
2 have a very small bias applied to the each base to improve the linearity of the
amplifier and reduce crossover distortion. This type of arrangement is referred to as
Cl
ass AB

since it has
characteristics of both Class A and Class B amplifiers. There are many additional amplifier classes
including C, D, E, F, G, H, and S, each of which is designed for specific operating conditions. The majority
of these additional amplifi
er types rely on frequency dependent input and/or output matching networks for
proper operation and are beyond the scope of this project. The operation of the Class B and Class AB
amplifier will be the focus of this project.

The Push
-
Pull amplifier is desi
gned to provide large output voltage swings and power gain. The two
transistors are arranged to allow one (NPN) to provide the current drive for positive swings and the other
(PNP) to provide the drive for the negative swings. The NPN transistor is said to

Source
current while the
PNP is said to
Sink

current. This Source
-
Sink relationship leads to the designation as a Push (source)
-

Pull
(sink) amplifier. The circuit in Figure 19
-
1 requires a non
-
BE(on)
) to be present
before either tr
ansistor turns on. This results in a distorted output signal since there is no output signal when
-

-
linearity (distortion) in the output signal
is referred to as
Crossover Distortion

since it occ
urs when the input signal crosses over from positive to
negative or negative to positive. The amplifier shown in Figure 19
-
2 is designed to prevent the crossover
distortion by maintaining the two transistors at the Edge of Conduction (EOC). Ideally, this a
rrangement
allows each transistor to respond to any non
-
zero input. The actual design however usually aims to maintain
the two transistors in a slightly forward biased operating state. Since each transistor has a small DC bias it
will dissipate a small DC
power even in the absence of an input signal. The Class AB designation is a
consequence of this power dissipation. Care must be taken in the Class AB design not to have a large
"reverse" crossover distortion by having both transistors operating for a given

input signal.




Figure 19
-

1: Class B Push
-
Pull Amplifier


Figure 19
-

2: Class AB Push
-
Pull Amplifier

Design:

1. Design a Class B amplifier as illustrated in Figure 19
-

a ± 10 V output swing. Determine the gain for the amplifier. You may as
sume you have between ± 15 V and
± 20 V power supplies available. Verify your design using PSPICE
®
.

2. Repeat step 1 for the Class AB amplifier illustrated in Figure 19
-
for the value of R. Be careful in your PSPICE
®

analysis to use a diode that can handle the DC current.

Lab Procedure:

1. Construct the amplifier designed in step 1 of the design procedure. Verify proper operation of the design.

2. Determine the amplifier voltage, current, and power gains along with th
e maximum symmetric
(effectively) output swing possible for your design.

3. Adjust the input signal to produce a 5 V peak output wave and measure the crossover distortion in terms
of input voltage levels and the phase angle(s) over which the output wave is

zero. Repeat for a 2 V peak
output wave.

4. Observe the loading affect by replacing R
L

output signal and comment on the loading affect.

5. Use computer control to record and plot the frequency resp
onse. Find the corner frequencies and
bandwidth.

6. Measure the input impedance seen by the source [look at the current from the function generator and the
node voltage at the base of the transistors] and the output impedance seen by the load resistor [loo
k at the
open circuit voltage and the current through and voltage across R
L

= 2 k

7. Repeat steps 1
-

6 for the Class AB amplifier designed in step 2 of the design procedure.

Questions:

1. Compare the output waveforms for the two circuits and discuss the similarities and differences especially
in terms of crossover distortion.

2. Th
e two designs do not have input or output coupling capacitors. Why are they omitted from these
designs? Would the operation be improved if they were included?

3. Could either (or both) of these amplifiers be designed using a single power supply? Justify yo
ur answer.

Do the two transistors have to be "matched", i.e. equal device parameters such as
BE(on)
, to
operate properly. Explain your answer.