1.0 OBJECTIVES To identify the characteristics of a Class A audio transformer-coupled power amplifier and Class AB complementary symmetry audio power amplifier. 2.0 MATERIALS REQUIRED a) b) c) d) e) f) g) h)
MAX Class A Amplifier Trainer MAX Complementary Symmetry Amplifier Trainer Digital Multimeter Oscilloscope Function Generator Screwdriver Wires/Jumpers
Loudspeaker 8Ω
3.0 THEORY
A class A power amplifier is defined as a power amplifier in which output current flows for the full-cycle (360°) of the input signal. In other words, the transistor remains forward biased throughout the input cycle.
1|Power Amplifiers
A schematic circuit of a series fed class A large signal amplifier using resistive
load Rc is shown below. The term “series fed” is derived from the fact that the load Rc is connected in series with the transistor output. The only difference between this circuit and the small-signal amplifier circuits considered previously is that the signals handled by the large-signal circuit are in the range of volts and the transistor used is a power transistor capable of operating in the range of a few watts. This circuit is seldom used for power amplification because of its poor collector efficiency but will give clear understanding of class A operation to the readers. The output characteristics with operating point Q are also shown. ICQ and VCEQ represent no signal collector current and collector-emitter voltage respectively. When ac input signal is applied, the operating point Q shifts up and down causing output current and voltage to vary about it. The output current increases to Ic max and falls to Ic min. Similarly, the collector-emitter voltage increases to Vce max and falls to Vce min.
2|Power Amplifiers
Power Distribution in Class A power amplifiers .
Input power from the collector supply V CC, Pin(dc) = VCC ICQ The power drawn from the collector supply is used in the following two components
• Power dissipated in collector load as heat, PRC (dc) = (ICQ)2 RC • Power supplied to the transistor, Ptr (dc) = Pin(dc) – PRC (dc) Power supplied to the transistor, P tr (dc) is further subdivided into ac power developed across the load resistor constituting ac power output and is given as 2
2
P out (ac) = (IC) RC = (Vce) / Rc where Ic and Vce are the rms values of collector current and Collector- emitter votage = {(I c max) ÷ √2}R C = V 2
2
2
(I c (peak-to-peak) Rc)/ 8 = V
CC
2
CE max
/ 2 Rc
(peak-to-peak) / 8 RC
Power dissipated, in the form of heat, by the transistor itself. The cause of power dissipation in transistor is explained below : Consider an N-P-N transistor. The potential difference across the depletion layer formed near the collector junction is called the barrier potential. This potential gives the P-region (base) slightly more energy than N-region (collector). Thus when electronics emitted from emitter cross the base junction and enter the collector region, they give up energy in the form of heat and it is this
energy
that
the
transistor
has
to
dissipate
to
the
surrounding.
With zero signal applied at the input of the class A power amplifier, ac power developed across the load reduces to zero and therefore all the power fed to the transistor is wasted in the form of heat. Thus, a transistor dissipates maximum power under zero-signal condition. Thus the device is cooler when delivering power to a load than with zero-signal condition.
3|Power Amplifiers
Since in class A operation, maximum power dissipation in the transistor occurs under zero-signal condition, the power dissipation capacity of a power transistor, for class A operation, must be at least equal to the zero-signal rating. Collector Efficiency: The collector efficiency of a transistor is given as
Efficiency = Average ac power output, P out (ac) / Average dc power input to the transistor P tr (dc), Power Efficiency: A measure of the ability of an active device to convert the dc power of supply
into the ac (signal) power delivered to the load is called the power or conversion or theoretical efficiency. By definition the efficiency is Efficiency = AC power delivered to the load, P out Pin (dc),
Now ac power delivered to the load,
Pout (ac) = (Ic (peak-to-peak) *Vce (peak-to-peak)) ÷ 8
4|Power Amplifiers
(ac) /
Total power drawn from dc supply
Maximum Power and Efficiency. If the operating point Q is set at the midpoint of the
maximum signal swing, the resulting maximum power condition may be achieved. Maximum VCE(peak-to-peak) = Vcc Maximum ICE(peak-to-peak) = Vcc ÷ RC maximum ac power developed across the load resistor,. 2
Pout (ac) max = 1/8 * Vcc/Rc * Vcc = V cc/8 Rc For the quiescent point Q, I CQ = (Vcc/Rc) ÷2 2
and dc power drawn from dc supply, P in (dc) max = Vcc ICQ = V cc/2Rc So maximum efficiency of an amplifier (class A power) is given as 2
2
Efficiency = Pout (ac) max / Pin (dc) max = (V cc/8 Rc) ÷ (V cc/2Rc) Class AB
This is the compromise of the bunch. Class AB operation has some of the best advantages of both Class A and Class B built-in. Its main benefits are sound quality comparable to that of Class A and efficiency similar to that of Class B. Most modern amp designs employ this topology. Its main characteristics are: In fact, many Class AB amps operate in Class A at lower output levels, again giving the best of both worlds. The output bias is set so that current flows in a specific output device for more than a half the signal cycle but less than the entire cycle. There is enough current flowing through each device to keep it operating so they respond instantly to input voltage demands. In the push-pull output stage, there is some overlap as each output device assists the other during the short transition, or crossover period from the positive to the negative half of the signal.
5|Power Amplifiers
There are many implementations of the Class AB design. A benefit is that the inherent nonlinearity of Class B designs is almost totally eliminated, while avoiding the heat-generating and wasteful inefficiencies of the Class A design. And as stated before, at some output levels, Class AB amps operate in Class A. It is this combination of good efficiency (around 50) with excellent linearity that makes class AB the most popular audio amplifier design. There are quite a few excellent Class AB amps available. This is the design I recommended for most general-use applications in home and car. Usually, parts choice rivals that of Class A amps, and dollar for dollar these are some of the best values in stereo amplification. There can be some variation in design principle, but generally these are well-designed amps since their function is very well-understood by audio designers.
6|Power Amplifiers
4.0 PROCEDURES/RESULTS Part 1 : Class-A Amplifier 1. Make sure that power supply is not connected. Carefully open the front panel of trainer board using a screwdriver and measure resistance of the primary and secondary coil of transformer T1. Table 1.1 : Transformer T1 (measured)
N1 Primary(Rp) 56
N2 Secondary(Rs) 64
2. Determine the turns ratio of transformer T1. The front panel of the trainer board is closed and fasten the screws.
Ratio of Transformer T1
= ²
√ = = √ = 0.935 : 1#
3. The circuit on trainer board as depicted in Figure 2 below has been completed. 4. The trainer has been switched on and the DC supply voltage has been measured. DC Supply Voltage (measured) = 8.90V
5. Measure the following parameters: i. Vbase of Q1 ii. Vcollector of Q1 iii. Vemmitter of Q1 iv. ICEQ Table 1.2 : Measured of Q1 (Transistor)
Parameter V base V collector V emitter I CEQ
7|Power Amplifiers
Measured 1.147V 8.54V 0.418V 4.89mA
Calculation 1.20V 9V 0.5V 5mA
%Error 4.417% 5.11% 3.8% 2.2%
Calculation :
Vcc (9) =
Vbase =
Vcollector = 9V
= 1.20V#
Vemitter = VB – VBE
=
IE =
= 1.20 – 0.7 = 0.5V
= 5mA
x 100% x 100% =
% error = Vbase
= 4.417 %# Vcollector =
x 100%
= 5.11%# VE =
x 100%
= 3.8%# 6. The values obtained in step 5 above should be consistent with your calculated values. This will indicate that your amplifier is working correctly. 7. A function generator is connected to the input of the amplifier. Select sine wave test signal of 1kHz. Adjust so that sine input is with the smallest amplitude possible. 8. Increase the 500Ω volume control potentiometer until a clean sine w ave output is obtained with minimum distortion. V out(max) has been measured. Measured of Vout(max) = 0.006V
9. The gain of this amplifier has been determined.
8|Power Amplifiers
The gain of this amplifier
re = Av = Av =
= 1.51#
10. We has been calculates the maximum efficiency of this amplifier. Maximum efficiency of this amplifier
η
= x 100%
= x100%
=
()() x 100% ()()
= 50%# 11. The input frequency from 200Hz until 20kHz has been varied and the sound of loudspeaker has been observed. OBSERVATION IN PART 1 :
When 200Hz = no sound made by the speaker Nevertheless, when after 200Hz until 20kHz = there is sound produced by the speaker
Figure 1 : Output waveform when diode is connected
9|Power Amplifiers
Part 2 : Class-AB Amplifier 1. The circuit on trainer board as shown in Figure 3 below has been completed. 2. The trainer has been turned on. Potentiometer has been fully rotated counterclockwise. We has been checked that 9V DC is supplied correctly. Frequency has been set to 1kHz. Oscilloscope on output has been tapped to monitor output waveform. 3. To a point just before the output waveform starts to flatten on either its top/bottom, the potentiometer R1 has been rotated clockwise. The peak-to-peak output voltage has been recorded. - R8//R9 = 50Ω 4. We are able to calculated the output power of this amplifier. Maximum efficiency of this amplifier
= ()
Po(max) = Po
= 1.266W# 5. The potentiometer R1 has been adjusted counter-clockwise for a low and comfortable listening volume. 6. A wire jumper has been repeatedly connected and disconnected shorting anode of diode D1 with cathode of diode D2. We have been listened to the output sound and also the output waveform has been observed during this process. The waveform has been recorded. We are also able to describe on the type of sound that was produced.
The sound produced become sharper then the sound before the diode has been short circuit.
7. The potentiometer R1 has been adjusted clockwise to increase the volume to the point until the output waveform starts to distorted. 8. The instruction in step 6 has been performed once again.
When the output waveform distorted, the sound produced become louder and when the diode D1 and D2 has been short circuit the sound become sharper.
10 | P o w e r A m p l i f i e r s
9. At high volume or low volume, the distorted sound has been determined where there is more pronounced. We are able to describe the output waveform that has been recorded.
The distorted sound when its at the high volume.
10. We are also able to calculated the maximum efficiency of this amplifier. Maximum efficiency of this amplifier
Pin = Vcc x
() = ()
=
=
η=
= 78.56%#
= 1.611W
Figure 2 : Output waveform when diode is connected
11 | P o w e r A m p l i f i e r s
Figure 3 : Output waveform when diode in short condition
12 | P o w e r A m p l i f i e r s
5.0 DISCUSSION The experiment to identify the characteristic of class A audio transformer-coupled power amplifier and the class AB complementary symmetry audio power amplifier. In part 1, we do the experiment to observed the characteristics of class A transformercoupled power amplifier. After the circuit on the trainer board is completed, we measuring the Vbase of Q1, Vcollector of Q1 and V emitter of Q1. We can conclude that the measured values is approximately and closer to the of the calculated values as shown in Table 1. Then, we determine the value of I CEQ. In order to do that, we assume that I CEQ = IC = IE. From the calculation, we got the value of IE = 5Ma. So, with the assumption that we assume, we know that the value of ICEQ is 5Ma. The maximum efficiency that we got from the calculation is 50%. As we vary the input frequency from 200Hz until 20kHz, we observed that at the beginning(when using 200Hz) there is no sound produced. As the frequency increased, the sound that has been produced will become more high pitched. In part 2, we do the experiment to observed the characteristic of class AB complementary symmetry audio power amplifier. The peak-to-peak output voltage that we got is 2.4V. From the calculation, the output power that we got is 0.2W. At the circuit, we repeatedly connect and then disconnect a jumper wire shorting anode of diode D1 with cathode of diode D2. The sound that has been produced become more high pitched and the waveform that has been produced as in figure 1. Then, we adjusting the potentiometer R1 clockwise to increase the volume to the point until the output waveform has been distorted we repeated the previous step(connect and disconnect). The distorted sound is at the high volume and the waveform as in figure 2. The waveform that we observed is decreasing when the diode is in short circuit. From the both experiments in Part 1 and Part 2, we conclude that the type AB amplifier is more efficient. This is because the sound that has been produced by the speaker from the circuit in part 2 is high pitched and better compared to the sound that has been produced from the speaker in part 1. However, the class A power amplifier have its own advantages compared to class AB power amplifiers. The advantages of class A power amplifier are as follows:
13 | P o w e r A m p l i f i e r s
a) Class A power amplifier has no crossover distortion. b) The amplifying element of class A power amplifier is biased. c) The class A power amplifier is generally better high frequency and feedback loop stability. The disadvantages of class A power amplifier are as follows: a) Class A power amplifier has low power efficiency. b) Class A power amplifier is limited for cutoff frequency and saturation region. c) Waste more power because it does not contribute to an AC output power. d) Class A power amplifier is more expensive than Class AB power amplifier. 6.0 CONCLUSION From the both experiments, we can conclude that Class AB power amplifier is more efficient than Class A power amplifier. As the frequency is increased, the sound that has been produced from the speaker of Class AB power amplifier is more high pitched.
14 | P o w e r A m p l i f i e r s
