EE 212 Lab
Lab 11: Operational Amplifiers-Part III (Negative-Impedance
Converters and General Impedance Converters)
Note: 741 op-amps are more forgiving and work better for this
This lab demonstrates two novel uses of op-amps: negative impedance
converters (NICs) and general impedance converters (GICs). To understand
how these circuits work, you need to analyze them in sinusoidal steady-state
operation or using frequency-domain methods.
1. Negative-Impedance Converters
- The NIC shown in figure 1 uses a 356 op-amp and resistors to produce an input
impedance of Zin = -Z. Design and build a NIC with Zin
= -10kΩ. Choose the resistors R to keep the op-amp
supply current from exceeding its maximum for the rated range of input
voltages. Also keep in mind that the op-amp circuit only acts as a NIC
so long as it is not operating in saturation.
- Put a voltage source and two 10kΩ resistors
in series with your -10kΩ NIC and take measurements
to verify that the NIC really behaves as -10kΩ.
You'll want to take voltage measurements at points in the circuit other than the
op-amp 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 op-amps and five
impedances. The input impedance of a GIC is given by
Z1 Z3 Z5/(Z2 Z4).
- Build a GIC to simulate (act as) a 10kΩ
resistor using two 356 op-amps and
- Use this simulated resistor in series with a real 10kΩ
resistor to build a voltage divider as shown in figure 3. Drive the voltage
divider with a 1kHz, 1V peak-to-peak sine wave. Does the output look like
what you would expect?
- A typical use of GICs is to simulate inductors. If Z2
or Z4 is a capacitor and the other impedances are resistors,
then Zin 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 op-amps, capacitors
and resistors are relatively easy to build. Modify your GIC such that it acts like a
- Use your "ideal'' 10H inductor in the RL circuit shown in figure 4. Determine
the circuit's time constant τ. (One method of finding
τ is to use an input square wave of period 10τ
to represent a step input and measure
the 10% to 90% rise time from which τ 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