Inverting OP Amp

1. Construct the inverting amplifier according to the wiring diagram below.

Here is the data sheet for the 741 Op Amp

Use [math]R_1 = 1k\Omega[/math] and [math]R_2 = 10 k\Omega[/math] as starting values.

2. Insert a 0.1 [math]\mu[/math]F capacitor between ground and both Op Amp power supply input pins. The Power supply connections for the Op amp are not shown in the above circuit diagram, check the data sheet.

Gain measurements

1.) Measure the gain as a function of frequency between 100 Hz and 2 MHz for three values of [math]R_2[/math] = 10 k[math]\Omega[/math], 100 k[math]\Omega[/math], 1M[math]\Omega[/math]. Keep [math]R_1[/math] at [math]1k\Omega[/math].

I have used the following values of [math]R_1[/math] and [math]R_2[/math] (as was suggested by Dr Forrest at the lecture)

[math]R_1 = (10.02 \pm 0.02)\ k\Omega[/math]
a) [math]R_2 = (99.0 \pm 0.2)\ k\Omega[/math]
b) [math]R_2 = (198.5 \pm 0.2)\ k\Omega[/math]
c) [math]R_2 = (800.0 \pm 2.0)\ k\Omega[/math]

So my theoretical gain of OP Amp would be:

a) Gain1[math]= \frac{R_2}{R_1} = \frac{99.0 \pm 0.2}{10.02 \pm 0.02} = (9.88 \pm 0.03)[/math]

b) Gain2[math]= \frac{R_2}{R_1} = \frac{198.5 \pm 0.2}{10.02 \pm 0.02} = (19.81 \pm 0.04)[/math]

c) Gain3[math]= \frac{R_2}{R_1} = \frac{800.0 \pm 2.0}{10.02 \pm 0.02} = (79.84 \pm 0.26)[/math]

Below is my measurements and gain calculation for the case a) and

Below is my measurements and gain calculation for the case b) and

Below is my measurements and gain calculation for the case c) and

2.) Graph the above measurements with the Gain in units of decibels (dB) and with a logarithmic scale for the frequency axis.

Below my plot of gain as function of frequency. Here

[math]G_{dB} \left(\frac{V_{out}}{V_{in}}\right) = 20\cdot \log_{10} {\frac{V_{out}}{V_{in}}}[/math]

Here the error calculation as usual and for this specific case is:

[math]dG_{dB}(x) = \frac{\partial G_{dB}(x)}{\partial x}\cdot dx = \frac{20}{x\ \ln 10}\cdot dx[/math]

where [math]x[/math] and [math]dx[/math] are corresponding gain and error of gain from the tables above

Impedance

Input Impedance

Measure [math]R_{in}[/math] for the 10 fold and 100 fold amplifier at ~100 Hz and 10 kHz frequency.

I am going to measure the input and output impedance of my amplifier using the following equivalent circuit:

where the shaded region is my actual amplifier where I have replaced the total input and output impedance by [math]R_{inp}[/math] and [math]R_{out}[/math]. What is inside the shaded area is my real amplifier I have contracted before:

From equivalent circuit the input impedance are:

[math]R_{inp} = \frac{V_{inp}}{I_{inp}}[/math]

and from my real circuit:

[math]I_{inp} = \frac{V_{inp}-V_1}{R_1}[/math]

so finally my input impedance becomes:

[math]R_{inp} = \frac{V_{inp}}{V_{inp}-V_1}\ R_1[/math]

Below is the table with my measurements and input impedance calculations for four asked different cases

As we can see the input impedance equals the resistor value [math]R_1[/math] for low frequency [math]f=100\ Hz[/math] and increase up to [math]33\ k\Omega[/math] for high frequency [math]f=10\ kHz[/math].

Output Impedance

Measure [math]R_{out}[/math] for the 10 fold and 100 fold amplifier at ~100 Hz and 10 kHz frequency. Be sure to keep the output ([math]V_{out}[/math]) undistorted

From equivalent circuit the input impedance are:

[math]V_{out} = V - I_{out}\cdot R_{out}[/math]

and I am going to use the load resistor [math]R_L[/math] to measure the output circuit:

[math]I_{out} = \frac{V_{out}}{R_L}[/math]

[math]V_{io}[/math] and [math]I_{B}[/math]

Use the above equation and two measurements of [math]V_{out}[/math], [math]R_1[/math], and [math]R_2[/math] to extract [math]V_{io}[/math] and [math]I_B[/math].

measure [math]V_{out}[/math] for [math]R_1[/math] = 1 k[math]\Omega[/math], [math]R_2[/math] = 100 k[math]\Omega[/math], and[math] V_{in}[/math]=0 (grounded).
measure [math]V_{out}[/math] for [math]R_1[/math] = 10 k[math]\Omega[/math], [math]R_2[/math] = 1 M[math]\Omega[/math], and[math] V_{in}[/math]=0 (grounded).
You can now construct 2 equations with 2 unknowns [math]V_{io}[/math] and [math]I_B[/math].

[math]I_{io}[/math]

Now we will put in a pull up resistor as shown below.

Instead of the current [math]I_B[/math] we have the current [math]I_{io}[/math]

Use the same technique and resistors from the previous section to construct 2 equations and 2 unknowns and extract [math]I_{io}[/math], keep [math]V_{in}[/math]=0.

The offset Null Circuit

Construct the offset null circuit above.
Adjust the potentiometer to minimize [math]V_{out}[/math] with [math]V_{in}=0[/math].
Use a scope to measure the output noise.

Capacitors

Revert back to the pull up resistor

Capacitor in parallel with [math]R_2[/math]

Select a capacitor such that when [math]\omega[/math]= 10 kHz.
Add the capacitor in parallel to [math]R_2[/math] so you have the circuit shown above.
Use a pulse generator to input a sinusoidal voltage [math]V_{in}[/math]
Measure the Gain as a function of the [math]V_{in}[/math] frequency and plot it.

Capacitor in series with R_1

Select a capacitor such that when [math]\omega[/math]= 1 kHz.
Add the capacitor in series to [math]R_1[/math] so you have the circuit shown above.
Use a pulse generator to input a sinusoidal voltage [math]V_{in}[/math]
Measure the Gain as a function of the [math]V_{in}[/math] frequency and plot it.

Slew rate

Measure the slew and compare it to the factory spec.

Power Supply Rejection Ratio

Set V_{in} = 0.
Measure [math]V_{out}[/math] while changing [math]V_{cc}[/math]

Output voltage RMS noise [math]\Delta V_{out}^{RMS}[/math]

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Lab 23 RS

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