The purpose of this experiment was to investigate the effects of negative feedback on the operation of the multi-stage amplifier which we designed in the three previous experiments. The differential and voltage gain stages will then be combined to yield a large open-loop gain, differential input, single-ended output amplifier. By utilizing the series-shunt feedback configuration, we will hopefully be able to operate a non-inverting amplifier with a gain of ten.

Negative feedback was employed in the design of the amplifier because of the following properties:

1. Desensitized the gain.
2. Reduced non-linear distortion,
3. Reduced the effect of noise.
4. Controlled the input and output impedance.
5. Extended the bandwidth of the amplifier.

The trade-off for the above advantages was the reduction of the amplifier’s gain. In turn, as the circuit was designed in a series-shunt feedback configuration, the range of frequencies for which the amplifier was able to remain at its mid-band condition was also extended.

1. R_in5 = R_B5 || (b +1)(r_e5+R_E5) = 19.96K || (110+1)(25+222) = 11.55KW

2. A_d = (R_C4 || R_in5) / 2r_e4 = (5.05K || 11.55K) / (2*52.4) = 33.53V/V

3. A_v = A_dA_mb = (33.53*14.7) = 492.9V/V

4. f_L = [2*pi*C_E5*(r_e+R_E5)(b +1) || R_B5 || R_C4]^-1 = [2*pi*(31.7x10^-6)(25+222)(110+1) || 19.96K || 5.18K]^-1 = 5.45Hz

5. Measuring the potential at the two test points and taking their ratio resulted in a close-loop gain of 8.7V/V. (Gain = TP2/TP1 = 2.1/0.24 = 8.7V/V)

6. BETA = R₁ / R₁+R₂ = 1.957 / (1.957+17.86) = 0.0987

7. f_Lf = f_L / (1+A_v*BETA) = 5.45 / (1+492.9*0.0987) = 0.11Hz (Expected) f_Lf = 0 Hz (Measured)

8. f_Max = I_E5 / (2*pi*C_B6*V_P) = 1mA / [2*pi*(1.74nF)*1] = 91.51KHz (Expected) f_Max = 83.45KHz (Measured)

9. The rise time of the output pulse was measured to be 6m sec.

10. f_hf = 0.350 / t_r = 0.350 / 6m sec = 58.3KHz

11. R_inf = TP2*R_s / (TP1-TP2) = (300mV*101) / (0.44-0.30) = 216.43KW

 In this experiment, the negative feed-back loop of our circuit was examined and verified to be working properly. This stage of the experiment will ensure that our amplifier will be non-invertingand will have an overall gain equal to ten. This will set up the final experiment which will be to provide for enough current gain to drive a load, i.e. a speaker in our case. The values obtained in this lab were as expected and no major difficulties were encountered, except for a bad transistor in our second stage. With the gracious assistance of the lab instructor, this minor problem was overcome and the we were able to finish the experiment.