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# The objective of this lab was to experiment with a negative feedback amplifier and predict its gain without the feedback.

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Introduction

20ECES465

Electronics Laboratory

Section 001

11/04/03

Objective

The objective of this lab was to experiment with a negative feedback amplifier and predict its gain without the feedback.

Procedure

After connecting the circuit per specifications in part one of the lab manual and verifying 7the voltage input and output we manipulated RFEEDBACK2 in order to achieve a gain of 100 v/v at a frequency of 1 KHz.  We then made a screen dump of the Acircuit value.  Since we were experimenting in the real world we could not use the ideal values for the components.  Table 1 shows the ideal values called for in the lab report and the actual resistance values measured by a multimeter.

Table 1: Resistance values

 Ideal (Ω) Real (Ω) 12.28k 12.26k 78.7k 78.6k 1.85k 1.85k 8.77k 8.8k 12.28k 12.29k 78.7k 78.7k 8.77k 8.74k 1.75k 1.750k

We then swept the voltage in order to find the fH frequency.  We then solved Equation 1 to get the feedback factor H.  After

Middle  Bandwidth Extension  Results

The results of the experiment in step 1 and step 2 are below in Table 2 and Table 3 respectively.  The gain of the negative feedback circuit was designed to be 100 V/V.  The high frequency 3db point was 940Khz.  When feedback was removed from the circuit the gain increased to 1509 V/V and the high frequency 3db point decreased to 185 KHz.  The applications of this are discussed in the individual discussion section.

Table 2: Feedback circuit values (step 1)

 AV 100 v/v RFEEDBACK2 16.32kΩ fH 940KHz

Table 3: Non-Feedback circuit values (step 2)

 VI .01 V VO 15.09 V AV 1509 V/V RFEEDBACK2 INFINITY fH 185KHz

Grant’s Discussion

This time around we got to hook up a circuit on the breadboard which was actually fairly complicated and compact.  This circuit needed to cover a very small area because

Conclusion

When we hooked up our circuit and finally got it working, we determined the RFEEDBACK value by increasing a variable resistor until we obtained the desired gain of 100V/V.  We arrived at a resistor value of 16.32kΩ.  The high 3dB frequency we obtained was 940kHz.  The feedback factor we determined to be was .01.

For the second part, we obtained a voltage gain 1509V/V.  This is consistent with the larger gain, by a factor of (1+AH).  We determined the high 3dB frequency to be 185kHz, which is also consistent with the feedback increasing the high 3dB frequency by that same factor.

Negative feedback amplifiers are extremely useful in applications such as inputs to analysis software from thermocouples and RTD’s,  They are used because they have a very stable, reliable gain, as well as a low output resistance.

Conclusion

In this lab, we demonstrated the difference between feedback and non-feedback amplifiers.  The effect of the feedback circuit was to increase the bandwidth of operation of the amplifier.  The trade off is in overall gain.

This student written piece of work is one of many that can be found in our AS and A Level Electrical & Thermal Physics section.

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