Methods of Inter Stage Coupling

                     In multistage amplifier, the output signal of preceding stage is to be coupled to the input circuit of succeeding stage. For this inter-stage coupling, different types of coupling elements can be employed. Those are :

1. RC coupling
2. Transformer coupling
3. Direct coupling

RC coupling : 

                     In RC coupled amplifier using transistors the output signal of first-stage is coupled to the input of the next stage through coupling capacitor and resistive load at the output terminal of stage.

Two stage RC coupled amplifier using transistors

                      The coupling does not affect the quiescent point of the next stage since the coupling capcitor Cc blocks the dc voltage of the first stage from reaching the base of the second stage. The RC network is broadband in nature. Therefore, it gives a wide-band frequency response without peak at any frequency and hence used to cover a complete AF amplifier bands. However its frequency response drops off at very low frequencies due to coupling capacitors and also at high frequencies due to shunt capacitors such as stray capacitance.

Frequency response of RC coupled amplifier

Transformer Coupling:

                        

                        In transformer coupled amplifier using transistors the output signal of first stage is coupled to the input of the next stage through an impedance matching transformer.

Two stage transformer coupled amplifier using transistors

This type of coupling is used to match the impedance between output and input cascaded stage. Usually, it is used to match larger output resistance of AF power amplifier to a low impedance load like loudspeaker. As we know, transformer blocks dc, providing dc isolation between the two stages. Therefore, transformer coupling does not affect the quiescent point of the next stage.

              Frequency response of transformer coupled amplifier is poor in comparison with that of an RC coupled amplifier. Its leakage inductance and inter-winding capacitances does not allow amplifier to amplify the signals of different frequencies equally well. Inter-winding capacitance of the transformer coupled may give rise resonance at certain frequency which makes amplifier to give very high gain at that frequency. By putting shunting capacitors across each winding of the transformer, we can get resonance at any desired RF frequency. Such amplifiers are called tuned voltage amplifiers.

     

              These provide high gain at the desired of frequency, i.e. they amplify selective frequencies. For this reason, the transformer-coupled amplifiers are used in radio and TV receivers for amplifying RF signals.

              As dc resistance of the transformer winding is very low, almost all dc voltage applied by Vcc is available at the collector. Due to the absence of collector resistance it also eliminates unnecessary power loss in the resistor.

Frequency response of transformer coupled amplifier

Direct Coupling : 

              In direct coupled amplifier using transistors the output signal of first stage is directly connected to the input of the next stage, This direct coupling allows the quiescent dc collector current of first stage to pass through base of the next stage, affecting its biasing conditions.

Two stage directly coupled amplifier using transistors

                 Due to absence of RC components, its low frequency response is good but at higher frequencies shunting capacitors such as stray capacitances reduce the gain of the amplifier.

                 The transistor parameters such as Vbe and β change with temperature causing the collector current and voltage to change. Because of direct coupling these changes appear at the base of the next stage, and hence in the output is called drift and it is serious problem in the direct coupled amplifiers.

Frequency response of direct coupled amplifier

Single Stage Amplifier

 Introduction : 

               We have seen that V-I characteristics of an active device such as BJT are non-linear. The analysis of a non-linear device is complex. Thus to simplify the analysis of the BJT, its operation is restricted to the linear V-I characteristics around the Q-point i.e. in the active region. This approximation is possible only with small input signals. The term small signal amplifier refers to the use of signal that takes up a relatively small percentage of an amplifier's operational range. With small input signals the transistor can be replaced with small signal linear model. This model is also called small signal equivalent circuit.

               We know that the reactance of the capacitance is inversely proportional to the frequency, Zc = 1/2pifC. Thus for low frequencies the reactances of junction capacitances of the transistor are very high. Since these junction reactances appear in parallel with junctions, their effect is ignored at low frequencies and transistor analysis is further simplifies.

Small Signal Low Frequency Transistor Amplifier Circuits:

                An amplifier is used to increase the signal level; i.e. the amplifier is used to get a larger signal output from a small signal input. We will assume a sinusoidal signal at the input of the amplifier. At the output, signal must remain sinusoidal in waveform, with frequency same as that of the input.

                To make the transistor work as an amplifier, it is to be biased to operate in the active region, i.e. base-emitter junction is to be forward biased, while base-collector junction to be reversed biased.

                Let us consider the common emitter amplifier circuit using self bias or voltage divider bias as shown below

                 In the absence of input signal, only dc voltage are present in the circuit. This is known as zero-signal or no-signal condition or quiescent condition for the amplifier. The dc collector-emitter voltage, Vce, the dc collector current Ic and dc base current Ib is the quiescent operating point for the amplifier. On this dc quiescent operating point, we superimpose ac signal by application of ac sinusoidal voltage at the input. Due to this base current varies sinusoidally.
                 Since the transistor is biased to operate in the active region, the output is linearly proportional to the input. The output current i.e. the collector current is β times larger than the input base current in common emitter configuration. Hence the collector current will also vary sinusoidally about its quiescent value, Icq. The output voltage will also vary sinusoidal.
                 The variations in the collector current and the voltage between collector and emitter due to change in the base current are shown graphically with the help of load line in below fig.

Graphical representation of base current, collector current, and

collector-emitter voltage awings

The collector current varies above and below its Q point value in-ohase with the base current, and the collector-to-emitter voltage varies above and below its Q point value 180 degrees out-of-phase with the base voltage.

                  When one cycle of input is completed, one cycle of output will also be completed. This means the frequency of output sinusoidal is the same as the frequency of input sinusoid. Thus in the amplification process, frequency of the output signal does not change, only the magnitude of the output is larger than that of the input.