Power Amplifier: Circuit and Operation

Chapter 2: High Power Amplifiers

Sub point 2.3

Circuit,  operation,  input  /output waveforms,  efficiency  and  power equations  of  Single  Stage  Class  A, Class  B,  Class  AB  and  Class  C  Power amplifier.

Circuit and operation of power amplifier

Class A Power Amplifier:

(Same circuit is used for Class B, Class AB and Class C power amplifier)

 

 

OR

 

Figure: Power amplifier (Common for class A, class B, class AB, class C)

Figure shows circuit diagram of class A power amplifier. Class A power amplifier is basically a common emitter (CE) amplifier with resistor or transformer as a load. The major difference between CE amplifier and power amplifier is the signals handled by the power amplifier and the transistor used is a power transistor which is capable of handling large power.

1. Transistor: In active region transistor can be used as an amplifier (Collector-Base junction (CB) reverse bias and Emitter-Base junction (EB) forward bias). The property of transistor is to provide the phase shift of 180­0, hence the output of amplifier is out of phase to the input signal.
2. Resistors R1 & R2: these resistors are used to provide voltage divider bias arrangement so as to operate transistor in active region.
3. Resistor Rc and Re: The output of circuit is taken across collector resistor Rc with respect to the ground. The function of emitter resistor Re is to stabilize the transistor. stabilization of transistor is necessary as it stabilizes the operating point of transistor and make collector current Ic independent of temperature changes. If transistor is not properly stabilized the there is possibility of thermal runway (Thermal runway is the self destruction of unstabilized transistor.
4. Capacitors Cin, and Ce: The property of capacitor is to block DC quantities and pass AC quantities. As amplifier is used to amplify AC i.e. analog signals, DC is unwanted in the circuit, it is blocked by capacitor Cin. Emitter capacitor Ce is used to provide low reluctance path to amplified signals.

For the resistive load arrangement, single-ended transistor is used in which its output stage with the resistive load connected directly to the Collector terminal. When the transistor turns ON, it sinks the output current through the Collector. The efficiency of class A power amplifier is very low (less than 30%) and will delivers small power outputs for a large drain on the DC power supply. Heat sinks are needed in power amplifiers because it passes the same load current even when no input signal is applied.

For transformer load arrangement, output transformer improves the efficiency of the amplifier by matching the impedance of the load with that of the amplifiers output impedance. By using transformer having suitable turns ratio, we can get up to 40% efficiency.

 

 Output wave forms of Class A, Class B, Class AB and Class C power amplifiers:

 

 

Figure: output wave form of class A power amplifier

 

Figure: output wave form of class AB power amplifier

Figure: output wave form of class C power amplifier

• Class B Push-Pull Power Amplifier:

 

 

  Figure: Class B Push-Pull Power Amplifier

Figure shows class B push-pull amplifier in which two transistors Q1 and Q2 are connected back to back. These two transistors are used for input and output respectively. Both transistors operates class B operation where transistor conducts only for half cycle of input signal. This type of output signal gives large distortion.  To avoid this distortion, two transistors connected in push-pull arrangement. One conducts in positive half cycle and other conducts in negative half cycle respectively.

Transformer T1 is called as input transformer and is called phase splitter which produces two signals which are 180⁰ out of phase with each other. Transformer T2 is called output transformer and is used to couple the AC output signal from the collector to the load. If no input signal is given to the circuit, both the transistor Q1 and Q2 are operates in cut-off and there is no current drawn from VCC.

Now, during positive half cycle, the base of Q1 is positive and Q2 is negative. This will turns ON transistor Q1 and turns OFF transistor Q2. At the output, we get half cycle. During negative half cycle, transistor Q1 turns OFF and Q2 conducts and we get another half cycle at the output.

At a time only one transistor is ON and processes one half cycle. The function of transformer T2 i.e. output transformer is to produce one complete full cycle by combining two half cycles.

• Complementary symmetry class B push-pull amplifier

 

 Figure: Complementary Symmetry Class B Push-Pull Amplifier

        Figure shows Complementary symmetry class B push-pull amplifier in which two transistors (NPN & PNP) are used in order to form complementary arrangement. The biasing conditions used for both transistors are same so they are symmetrical. Resistors R1, R2 and VCC are used to form voltage divider bias circuit of transistors. Both transistors conduct for 180° as it is class B amplifier. Whenever one transistor is ON other push to be OFF so the name push pull.

 

• Working:

Input signal Vin is applied to both the transistor through input capacitor.

Ø During positive half cycle of input:

· The base of the transistors NPN & PNP is positive.

· As a result of this NPN conducts & PNP remains OFF.

· So we get half cycle in the output.

Ø During negative half cycle of input:

· The base of the transistors NPN & PNP is negative.

· As a result of this PNP conducts & NPN remains OFF.

· So we get remaining half cycle in the output.

 

• What is crossover distortion and how to avoid it?

 Cross over distortion occurs in Class B push pull Amplifier. In the push-pull configuration, the two identical transistors turns ON sequentially, one after the other and the output produced will be the combination of both. When the signal changes or crosses over from one transistor to the other transistor at the zero voltage point, it produces distortion in the output wave shape. The base emitter junction of transistor should cross 0.7v to turn ON. The time taken for a transistor to get turn ON from OFF state or to get OFF from ON state is called the transition period which leads to crossover distortion.

Cross over distortion can be removed or avoided if the conduction of the transistor is more than one half cycles, due to which both the transistors will not be turned OFF at the same time.