This document discusses different classes of power amplifiers, including class A, class B, class AB, and push-pull amplifiers. It provides details on the operating principles, biasing, power efficiency, and output characteristics of each type. Key points include: Class A amplifiers have output current flowing for the full input cycle, leading to low efficiency. Class B amplifiers only conduct for half the input cycle. Class AB provides a small amount of bias to increase conduction. Push-pull amplifiers use two transistors connected out of phase to increase power and gain.

1. NATIONAL COLLEGE OF SCIENCE AND TECHNOLOGY
Amafel Bldg. Aguinaldo Highway Dasmariñas City, Cavite
ASSIGNMENT 1
CLASSES OF AMPLIFIER
Olaño, Reymart September 01, 2011
Electronics 3/BSECE 41A1 Score:
Engr. Grace Ramones
Instructor

2. 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.
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.
Power Distribution in Class A power amplifiers.
Input power from the collector supply VCC, 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

3. • Power supplied to the transistor, Ptr (dc) = Pin(dc) – PRC (dc. Power supplied to the transistor, Ptr
(dc) is further subdivided into ac power developed across the load resistor constituting ac power output
and is given as P out (ac) = (IC)2 RC = (Vce)2/ Rc where Ic and Vce are the rms values of collector current
and Collector- emitter votage = {(Ic max)2÷ √2}RC = V2 CE max / 2 Rc
(I2 c (peak-to-peak) Rc)/ 8 = V2CC (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.
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, Pout (ac) / Average dc power input to the transistor Ptr (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, Pout (ac) / Total power drawn from dc supply Pin (dc),
Now ac power delivered to the load,
Pout (ac) = (Ic (peak-to-peak) *Vce (peak-to-peak)) ÷ 8
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.

4. Maximum VCE(peak-to-peak) = Vcc
Maximum ICE(peak-to-peak) = Vcc ÷ RC
we have maximum ac power developed across the load resistor,.
Pout (ac) max = 1/8 * Vcc/Rc * Vcc = V2cc/8 Rc
For the quiescent point Q, ICQ = (Vcc/Rc) ÷2
and dc power drawn from dc supply, Pin (dc) max = Vcc ICQ = V2cc/2Rc
So maximum efficiency of an amplifier (class A power) is given as
Efficiency = Pout (ac) max / Pin (dc) max = (V2cc/8 Rc) ÷ (V2cc/2Rc)
This is the maximum percent efficiency for a series-fed class A power amplifier. Since this maximum
efficiency will occur only under ideal conditions and for the maximum ac signal swings, most series-fed
class A power amplifiers have power efficiencies much less than 25%.

5. CLASS-B-POWER-AMPLIFIER-OPERATION
In class B operation the transistor is so biased that zero-signal collector current is zero. Hence class B
operation does not need any biasing system. The operating point is set at cut-off. It remains forward
biased for only half cycle of the input signal.i.e its conduction angle is 180 degree.
As illustrated in figure, during the positive half cycle of the input ac signal, the circuit is forward biased
and, therefore, collector current flows. On the other hand, during negative half cycle of the input ac
“signal, the circuit is reverse biased and no collector current flows.
Power and Efficiency Calculations of class B operation. Input dc power, Pin (dc) = VCC ICCwhere ldc
is the average or direct current taken from the collector supply.
If Ic max is the maximum or peak value of collector or output current, then
ldc = lc max / ∏
RMS value of collector current, Ic rms = Ic max/√2
RMS value of output voltage, Vrms = VCC/√2
Hence output power during half cycle, Pout (ac) = (Ic max VCC)/ 4
In above expression factor 1/2 is used because power is developed during one half cycle only.
DC power loss in load, PRc(dc) = (I2 dc Rc = Ic max/∏) Rc . ..
DC power loss in collector region or transistor, = Pin dc - PRc(dc) – Pout (ac)
Overall efficiency, noverall = Pout (ac)/ Pin dc = 0.785 or 78.5%

6. CLASS-AB-POWER-AMPLIFIER-OPERATION
In class AB power amplifiers, the biasing circuit is so adjusted that the operating point Q lies near the
cut-off voltage. During a small portion of negative half cycle and for complete positive half cycle of the
signal, the input circuit remains forward biased-and hence collector current flows. But during a small
portion (less than half cycle) of the negative cycle”‘ the input circuit is reverse biased and, therefore, no
collector current flows during this period. Class AB operation needs a push-pull connection to achieve a
full output cycle.
In AB diagram, a small amount of bias current is flowing through the valve. For the output valves in a
typical class AB guitar amplifier, this would amount to around 30-40mA, with peaks of approximately
250-300mA.
In the push-pull output stage, there is a little overlap as each valve assists it's neighbour during a short
transition, or crossover period.
Many larger guitar amplifiers are class AB, and we'll find out why a little later on.

7. PUSH-PULL AMPLIFIERS
One use of phase splitters is to provide input signals to a single-stage amplifier that uses twotransistors.
These transistors are configured in such a way that the two outputs, 180º out of phase witheach other,
combine. This allows more gain than one transistor could supply by itself. This "push-pull"amplifier is
used where high power output and good fidelity are needed: receiver output stages, publicaddress
amplifiers, and AM modulators, for example.The circuit shown in figure 1-29 is a class A transistor push-
pull amplifier, but class AB or class Boperations can be used. Class operations were discussed in an
earlier topic. The phase splitter for thisamplifier is the transformer T1, although one of the phase
splitters shown earlier in this topic could beused. R1 provides the proper bias for Q1 and Q2. The tapped
secondary of T1 develops the two inputsignals for the bases of Q1 and Q2. Half of the original input
signal will be amplified by Q-1, the otherhalf by Q-2. T2 combines (couples) the amplified output signal
to the speaker and provides impedancematching.
Figure 1-29.—Class A transistor push-pull amplifier.