New Mexico Tech
EE321L - Analog Electronics
Fall 2013
Laboratory Exercise 1
Amplifiers, Input/ Output Resistance, Cascades and Frequency Response
Pre-Lab
1.
Find the LM741 data sheet, for example on www.digikey.com.
2.
Design the amplifier in question 1.
3.
Design the amplifiers in question 5, derive an expression for Avo of the cascade, and compute
the expected value.
4.
Design the amplifiers in question 6.
5.
Determine what bandwidth is and how to compute it based on the Gain-Bandwidth-Product
of the amplifier.
Input and Output Resistance
1.
Using a LM 741 op-amp, build an inverting voltage amplifier with a gain of approximately 100, and input and output resistances of 1 kΩ (Hint: the circuit is a regular inverting
amplifier followed by a series resistor). Measure the resistors and compute an actual
expected gain, Avo.
2.
Attach the function generator directly to the input, and the output directly to a voltage-meter
of the oscilloscope input. Plot the output as a function of the input (use the function
generator’s DC Offset by selecting the buttons: Waveform -> More -> DC -> Parameter, and
then entering the values you desire) for several different input voltages. Start at and
increment by 10mV. What does this imply about the source (function generator) and load
(meter or oscilloscope) resistances?
NOTE: Press the channel button and set output to “on” to output a signal. It is good habit to
set the output to “off” when adjusting the circuit or no longer using the waveform.
3.
Measure the input and output resistance of the amplifier using DC Signals and a 1 kΩ
potentiometer as part of a voltage divider. You can use the function generator’s DC offset
with the potentiometer in series at the input. Repeat with the potentiometer in series with
the output. Set the function generator to have a DC voltage of 80 mV. Adjust the
potentiometer until the potentiometer’s output voltage is half the input voltage. Once
finished remove the potentiometer and measure its resistance value. Compare the results to
the actual value of the 1 kΩ resistors. Is the 50 Ω output resistance from the function
generator negligible?
NOTE: The function generator describes all voltages, including DC, as peak-peak values,
therefore if you view the 80mV signal on the oscilloscope, it will display a voltage around
160 mV.
Hint: Make sure to place the potentiometer as “r1” or the voltage divider will not act as
desired.
New Mexico Tech
EE321L - Analog Electronics
Fall 2013
Frequency Response
4.
Measure the frequency response of the amplifier. Use a small-amplitude sinusoidal input,
gradually increasing the frequency until the gain has dropped significantly (by at least a
factor of 10.) Plot the gain and phase difference between input and output as a function of
frequency. Compare to your expectations. Is the low frequency gain what you expect? What
is the bandwidth? Compare your bandwidth values by searching the datasheet for its
declared bandwidth and/or Gain-Bandwidth-Product (GBWP). Once found, divide the GBWP
(should be between 0.5Mhz and 1.5MHz) by the gain you are using. Increase the Frequency
until a gain of 1 is reached. Does this value agree with the datasheet? Would you want to use
the LM741 for audio amplification knowing that the phase of low frequencies and high
frequencies are not constant?
NOTE: At high frequencies you must use a small amplitude input signal. If you notice any
distortion of the output causing it to look triangular, reduce the input signal amplitude.
Cascaded Amplifiers
5.
Build a two-stage amplifier cascade consisting of two inverting amplifier, each with a gain of
10 and each with input and output resistances of 1k. Measure Avo of the cascade.
6.
Modify your two-stage amplifier circuit (keeping the feedback resistance values identical) to
produce an Avo of approximately 100. Measure the new gain and confirm it is 100.
7.
What is the bandwidth of the cascade? You can measure this without plotting the full
frequency response. Compare this measure bandwidth of the two-stage amplifier with the
theoretical value you computed based on the bandwidth of the single-stage amplifier.