Download AUDIO POWER AMPLIFIERS Introduction

AUDIO POWER AMPLIFIERS
Introduction
In this laboratory, some important aspects of different power amplifiers configurations will be
considered.
Theory
An amplifier designed to deliver electric power to a desired load is known as a power
amplifier. Power amplifiers find applications in transmitter, servomotor amplifiers, industrial
control circuits, and audio amplifiers. In general, power amplifiers designed to deliver the
maximum power output at the highest efficiency. Since power amplifiers inherently involve
excursions in voltage and current, the transistor may operated in the non-linear regions of
the characteristic curve resulting distortion in the output. Furthermore, the transistor
subjected to large values of current and voltage, thermal instability may become a problem
and thus the power amplifier must biased to guard against thermal runaway.
Depending on their operation, power amplifiers can be grouped into four main classes:
- CLASS A operation: 360o conduction angle
- CLASS AB operation: 180o conduction angle
- CLASS B operation: less than 360o, more than 180o conduction angle
- CLASS C operation: less than 180o conduction angle (fixed drive)
The output signal as result of a sinusoidal input signal for each of the four classes is shown in
Figure 1. In class A operation, the entire input signal is reproduced faithfully at the output
resulting minimum distortion. The power delivered by the power supply is constant and not
affected by input signal. This means that, power being dissipated by the circuit even through
no signal is present. Further more, the maximum lower dissipated in the transistor under up
signal condition. Figure 2 shows the typical circuit used for class A power amplification.
In class AB operation output (collector) current flows for more than half of the input signal
cycle. Hence, more than half of the signal is amplify and appears at the output.
Figure 1: Transistor collector currents for different amplifier classes: (a)Class A, (b)Class B, (c)Class
AB, (d)Class C
Figure 2: Direct coupled load amplifier (Class A)
In class B operation, exactly half of the input signal appears amplified at the output.
Transistor is biased such that Ic =0. Therefore the amplifier dissipates power only when it is
being used to amplify signal (input signal present). Hence, the efficiency is higher but the
distortions are considerable. In order to obtain high efficiency and low distortion, circuit
shown in Figure 3 is used. This is known as PUSH PULL amplifier. In positive half cycle one
transistor provides output current while in negative half cycle other transistor provides
output current. By this way output current is continuous.
Figure 3: PUSH-PULL amplifier (Class B)
In class C operation, somewhat less than half of the input signal appears amplified at the
output. The output signal waveform is high distorted and rich in harmonic. Generally, in class
C amplifier, load is a tuned circuit, which selects the fundamental or the desired harmonic
rejecting all other frequency components. Efficiency in class C amplifier is the highest. These
generally used to amplify radio-frequency single in transmitters.
One of the most important parameters of power amplification is efficiency, which is defined
as the ratio of the usable AC output power to the total power consumed by the output
stage. It is important not only for the reasons of economy but for the inconvenience of
dissipating the wasted power. Consider the configuration in Figure 4.
Figure 4: Basic amplifier structure (Class-A)
Assuming a sinusoidal current with peak amplitude I, then the AC power delivered to the
load is:
1
𝑃𝐿,𝐴𝐶 = 𝐼 2 𝑅𝐿
2
Input power from supply:
𝑃𝑆 = 𝑉𝐶𝐶 𝐼𝐶𝑄
DC power delivered to the load:
2
𝑃𝐿,𝐷𝐶 = 𝐼𝐶𝑄
𝑅𝐿
Power dissipated in the device:
2
𝑃𝐷 = 𝑃𝑆 − 𝑃𝐿,𝐷𝐶 − 𝑃𝐿,𝐴𝐶 = 𝑉𝐶𝐶 𝐼𝐶𝑄 − 𝐼𝐶𝑄
𝑅𝐿 −
𝑅𝐿 2
𝐼
2
Assuming the device can be operated from cut-off to saturation (maximum efficiency case):
𝐼=
𝑃𝐿,𝐴𝐶
𝑉𝐶𝐶
2𝑅𝐿
𝑉𝐶𝐶 2
=
8𝑅𝐿
𝑉𝐶𝐶 2
𝑃𝑆 =
2𝑅𝐿
𝑃𝐿,𝐷𝐶
𝑉𝐶𝐶 2
=
4𝑅𝐿
𝑉𝐶𝐶 2
𝑃𝐷 =
8𝑅𝐿
𝜌 = 𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 =
𝑃𝐿,𝐴𝐶 1
= = 25%
𝑃𝑆
4
It should be noted that this efficiency figure applies only to maximum amplitude sinusoidal
output and is higher than the average efficiency achieved with a typical practical signal such
as speech, music etc. This configuration has the property that the device current never falls
completely to zero except at the very peak of the signal excursion.
Figure 5: Class-B Push-Pull amplifier structure
Consider the configuration in Figure 8. In this configuration, two transistors are used and
only one of them is conducting at a time. Transistors are completely identical except that
one of them is NPN and the other is PNP type. The name of this configuration comes from
the fact that, for positive cycles NPN “pushes away” the current where as for negative cycles
PNP “pulls” the current from the load. For zero signal, both devices are cut-off. Assuming
transistors have 0V base-emitter drop, for positive cycles T1 (NPN) conducts and produces a
half wave rectified waveform. For negative cycles, however, T2 conducts. Resulting half
wave rectified waveforms are added at the load and produce a sinusoid load voltage. Note
that average DC current in each device is IC=I/π which is equal to the DC value of a half wave
rectified sinusoid.
For this configuration;
𝑃𝐿,𝐴𝐶
𝑉𝐶𝐶 2
=
2𝑅𝐿
𝑃𝑆 =
2𝐼𝑉𝐶𝐶
𝜋
𝜌=
𝑃𝐿,𝐴𝐶
= 78.5%
𝑃𝑆
When both of the transistors are unbiased, they will not conduct until the voltage across
their base-emitter terminal exceeds an opening voltage (0.7 V for silicon transistors). This
condition gives rise to a kind of distortion called cross-over distortion leading to the creation
of odd harmonics, which is not permissible in good quality, high fidelity (HI-FI) equipment.
Some precautions must be taken to prevent this occurrence. Most of the time, transistors
are slightly biased just at the threshold of conduction to overcome that problem.
Keep in mind that the input resistance of a power amplifier cannot be determined using
linear models as the input is usually high enough to violate the small signal condition that
has to be satisfied in order to use the linear models.
Another important issue when dealing with power amplifiers is heat dissipation. Transistors
used in output stages may have to withstand currents in the ampere range and thus large
amounts of power may be dissipated on them. This dissipated power is converted into heat.
To prevent the transistor temperature from rising above specified maximum levels, we can
use heatsinks. Heatsink in the general sense is an environment or object that dissipates
power from an object using thermal contact. Transistor heatsinks are usually large metal
surfaces that are bolted to the case. The high surface area and the thermal conductivity of
the metal helps decrease the junction temperature of the transistor efficiently.
Preliminary work
1. For the circuit in Figure 4, assume that the output voltage is purely sinusoidal, i.e.
VE=VEQ+V1coswt. By integrating the product of emitter current and emitter voltage
over one cycle, show that PL=VEQIEQ+(V1I1)/2 where I1 is the peak value of emitter
current.
2. Consider the amplifier configuration given in Figure 6 below. Assume that R1 and R2
values are arranged such that the maximum peak to peak signal excursion is obtained
at the output. Find the value of RC2 so that the load resistances do not burn during
the operation.
Figure 6: Class-A amplifier structure
3. In the circuit given in Figure 7, the output is a complementary emitter follower with
bases connected together and operating in strict Class-B. Calculate the minimum
value of the load resistance such that peak collector current is less than 100 mA.
Assuming Vs = 3coswt, plot the load voltage, and calculate its peak value.
Figure 7: Class-B amplifier structure
4. The operation of the output transistors is class-AB in the circuit shown in Figure 8,
that is, under zero input condition, a small collector current flows due to base bias.
Calculate R such that the quiescent collector current is aroun 7.5 mA. What are the
advantages and disadvantages of having 10 Ω power resistors?
Figure 8: Class-AB amplifier structure
Experiment
1. Construct the circuit given in Figure 7 with RL =100 Ω.
a. Measure the DC voltages of each transistor (i.e. VB, VC, VE) while the input is
disconnected (zero input).
b. Increase the input signal up to a point where there is no distortion at 1 kHz,
and record the input signal waveform, load voltage, and input versus output
voltage characteristics. (Note: IC=I/π)
c. Measure appropriate variables to calculate PL,AC, PS, and efficiency.
2. Build a VEE program to increase the input signal from zero to the point you found in
the previous step, and plot the output voltage of the class-B amplifier. Use the same
circuit of Figure 7 with RL=1 kΩ.
3. Construct the circuit of Figure 9 with a BD135 transistor.
a. Measure and record DC bias voltages (VE, VB, VC).
b. Calculate the value of DC bias current (IE=VE/RE).
c. Use the signal generator to give a sinusoidal input wave at 10 kHz. Obtain the
largest undistorted output signal. Measure and record these input and output
voltages.
d. Using the measured values, calculate the input power, output power, and
efficiency.
e. Reduce the input signal to one-half level, and measure and record the input
and output voltages.
f. Recalculate the input power, output power, and efficieny for the half input
voltage.
AUDIO POWER AMPLIFIERS
NAMES:
1.
2.
SECTION:
1.
a) (NPN) VB = …………… VC = …………… VE = ……………
(PNP) VB = …………… VC = …………… VE = ……………
b) Plot Vin , Vout and Vout – Vin
c) PL,AC ……………
2.
PS = ……………
efficiency = ……………
Plot the VEE output (indicate the axis parameters and scales)
3.
a) VB = ……………
VE = ……………
VC = ……………
b) IE = ……………
c) Vi = ……………
d) Pi = ……………
e) Vi = ……………
f) Pi = ……………
Vo = ……………
Po = ……………
efficiency = ……………
Vo = ……………
Po = ……………
efficiency = ……………