In this experiment, we will be investigating the amplifier power stage. In order to be able to drive a load with a small impedance, such as a load speaker in our case, the amplifier has to contain an additional stage to provider for a current gain which will compliment the voltage gain provided for by the previous stage. Since the common-collector amplifier configuration provides for a large current gain, they will be used as the output stage of our operational amplifier. The output stage which we will examine in this experiment will be the low distortion class A and the high efficiency class B output stages. With these voltage and current gain stages combined, the required power to the load was provided and a linear operation obtained as demonstrated in the laboratory.

The output stage serves an important function since it provides the circuit with a low output resistance in order for the amplifier to deliver an output signal without losing any of the gain delivered by the input signal. The first two stage of the amplifier in this experiment were used to increase the voltage of the input signal. Because of the limitation in the voltage swing, the voltage gain was very small. As a result, an emitter-follower stage was implemented to generate more signal power in this circuit. With the addition of this second stage, the amplifier was able to match the low impedance load of the speaker. In addition, combining a class A power stage with that of a class B power stage provide the capability of correcting for crossover distortion. The class B power stage consisted of a complimentary pair of transistors which were connected in a manner which prevented them from conducting simultaneously. The class B stage also had the capability of achieving a power conversion efficiency than that of the class A power stage.

V_PMax = 1.3V

R_L = 1.01KW

I_sinkmax = V_PMax / R_L = 1.3 / 1.01K = 1.287mA

Part 3: (Class A DC Power Stage Measurements)

V_B10 = -0.284 V

V_E10 = -0.966 V

V_C10 = 4.99 V

I_E = (V_E10-V_EE) / 197.2 = (-0.966+4.99) / 197.2 = 20.41 mA

I_C = I_E*(b /b +1) = 20.41mA*(100/101) = 20.21 mA

P_diss = I_E²R_E + V_CE I_C = (20.41mA)2(195) + (5.966)*(20.21 mA) = 207.11 mW

I_B10 = I_E10 / (b +1) = 20.41 mA / 101 = 200 m A

R_Lmin = 140 W and V_peakout = 1.5 V

I_Lmax = V_peakout / R_Lmin = 1.5 V / 140 W = 10.71 mA

t_cutoff = 0.8 div * 0.2 ms/div = 160 m s

t_period% = 160 m s/ 1 ms = 16%

t_Theory = arcsin(Vt/VP) / (2*pi*1K) = arcsin(0.5/0.8) / (2*pi*1K) = 108 m s (This value is too low)*

*See conclusion for explanation

t_cutoff = 60 m s

t_period% = 60 m s/ 333 m s = 18%

t_Theory = arcsin(Vt/VP) / (2*pi*1K) = arcsin(0.5/0.8) / (2*pi*3K) = 35.8 m s (This value is also too low)*

*See conclusion for explanation

P_peak = V_CE*I_Epeak = (5-1.5)*(1.5/8) = 656 mW

P_avg = 0.5*P_peak = 328 mW

In this experiment, we gained valuable knowledge in the area of amplifier power stages, especially in the class A and class B power amplifiers. From this experiment, it can be observed that the the current gain of the power amplifier needed to be increased to provide for the required base voltage needed to run the class B power stage. From this experiment, it can also be observed that the class A power amplifier stage had the least amount of signal distortion; this was at the cost of low efficiency though. The class B power amplifier stage, in contrast, had high efficiency, but this also came at a cost in the form of cross-over distortion which occurred when one of the two transistors took over the operation. Upon the completion of the circuit, the amplifier was able to drive the low impedance speakers located on the cadet. The low values obtained in the experiment from parts 6 and 7 can be attributed to measurement errors. Also, values for V_p and V_t were assumptions and does not take into account the actual values of V_p and V_t from our circuit.