Experiment # $6$: Opamp Application Experiments

Goals:

The performance of the non$-$inverting and inverting voltage amplifiers will be examined.

The investigation will include the effect of feedback resistors on setting voltage gain, stability of gain with differing op amps, and input impedance and the concept of virtual ground.

Equipment:

Oscilloscope: Dual channel Digital Oscilloscope

Function Generator: Function Generator

Power Supply

$741,071$ or similar opamp

Breadboard

Resistors available in the laboratory

Part$-$A: The Non$-$inverting Voltage Amplifier Background Information:

The non$-$inverting voltage amplifier is based on series$-$parallel negative feedback. As the ideal voltage$-$controlled voltage source, this amplifier exhibits high input impedance, low output impedance, and stable voltage gain. The voltage gain is set by the two feedback resistors, Ri and Rf$.$

Schematics

Experimental Procedure:

The voltage gain of the non$-$inverting amplifier can be determined accurately from the feedback resistors $R_i$ and $R_f$ Calculate the voltage gains for the amplifier of Fig. $1$ for the $R_f$ values specified, and record them in Table $1.$

Assemble the circuit of Fig. $1$ using the $4k7$ resistor.

Set the generator to a $1$ kHz sine wave, $100$ millivolts peak.

Apply the generator to the amplifier. Measure and record the output voltage in Table $1.$ Also, compute the resulting experimental voltage gain and gain deviation.

Repeat step $4$ for the remaining Rfvalues in Table $1.$

For any given $R_i,R{f}_f$ combination, the voltage gain should be stable regardless of the precise op amp used, even if it is of an entirely different model. To verify this, first set $R{f}_f$ to $22k.$

Set the generator to a $1$ kHz sine wave, $100$ millivolts peak.Apply the generator to the amplifier. Measure and record the output voltage in Table $2.$ Also, compute the resulting experimental voltage gain and gain deviation.

Repeat step $8$ for two other op amps.

It is not practical to use an ohmmeter to determine the input impedance of an active circuit. Instead, input impedance can be found by utilizing the voltage divider effect. Modify the circuit by adding the extra input resistor as shown in Fig. $2.$

Set $R_f$ to $4k7.$

Set the generator to a $200$ Hz sine wave, $1$ volt peak.

Apply the generator to the amplifier. Use a DMM to measure and record the AC potential from $V_{in}$ to point X $($i$.$e$.,V_A,$ the voltage across the $100$ k $)$ in Table $3.$ Using KVL$,$ determine the voltage from point X to ground $(V_B)$ and record in Table $3($don$\text{'}$t forget to compensate for peak versus RMS readings$).$ Finally, compute the resulting input impedance by using the voltage divider rule. Note: If the DMM is not sensitive enough and registers $0$ volts for $V_A,$ it is safe to assume that $Z_{in}$ is considerably larger than the $100k$ sensing resistor.

Data Tables

Table $1$

$\backslash$table$[[R_f,$Theoritical Av$,$Vout,Experimental Av$,\%$ Deviation$],[4k7,,,,],[10k,,,,],[22k,,,,],[33k,,,,],[47k,,,,]]$

Table $2$

$\backslash$table$[[$Opamp$,$Theoritical Av$,$Vout,Experimental Av$,\%$ Deviation$],[1,,,,],[2,,,,],[3,,,,]]$

Table $3$

$\backslash$table$[[V_A,V_B,$Zin$],[,,]]$

Questions

What is the effect as $R_f$ is increased?