EE 212 Lab
Lab 11: Operational Amplifiers,
NegativeImpedance Converters and General Impedance Converters
Prelab 11
Note: 741 opamps are more forgiving and work better for
this experiment
This lab demonstrates two novel uses of opamps: negative impedance converters
(NICs) and general impedance converters (GICs). To understand how these circuits
work, you need to analyze them in sinusoidal steadystate operation or using
frequencydomain methods.
1. NegativeImpedance Converters

The NIC shown in figure 1 uses a 356 opamp and resistors to produce an input
impedance of Z_{in }= Z. Design and build a NIC with
Z_{in} = 10kW. Choose the resistors R
to keep the opamp supply current from exceeding its maximum for the rated
range of input voltages. Also keep in mind that the opamp circuit only acts
as a NIC so long as it is not operating in saturation.

Put a voltage source and two 10kW resistors in
series with your 10kW NIC and take measurements
to verify that the NIC really behaves as 10kW.
You'll want to take voltage measurements at points in the circuit other than
the opamp inputs to minimize ringing that can be caused by connection of
a probe.
2. General Impedance Converters

Figure 2 shows a GIC which is built using two opamps and five impedances.
The input impedance of a GIC is given by
Z_{1} Z_{3} Z_{5}/(Z_{2} Z_{4}).

Build a GIC to simulate (act as) a 10kW resistor
using two 356 opamps and five resistors.

Use this simulated resistor in series with a real
10kW resistor to build a voltage divider as shown
in figure 3. Drive the voltage divider with a 1kHz, 1V peaktopeak sine
wave. Does the output look like what you would expect?

A typical use of GICs is to simulate inductors. If Z_{2} or Z_{4
}is a capacitor and the other impedances are resistors, then
Z_{in} is the impedance of an inductor. GICs are used to simulate
inductors because it is virtually impossible to build an inductor as part
of an integrated circuit, whereas opamps, capacitors and resistors are
relatively easy to build. Modify your GIC such that it acts like a 10H inductor.

Use your "ideal'' 10H inductor in the RL circuit shown in figure 4. Determine
the circuit's time constant t. (One method of
finding t is to use an input square wave of period
10t to represent a step input and measure the
10% to 90% rise time from which t can be computed
as in lab 3.)

Show that this is the time constant you would expect if the inductor and
resistor were both ideal.

Increase the amplitude of the input voltage until the output becomes distorted.
Why does the response of the circuit with the simulated inductor depart from
the ideal response?
© Copyright 2003 New Mexico Institute of Mining and Technology