Download Pre-lab4 Problems

Physics 3330
Solutions Pre-lab 4
Spring 2013
Pre-lab4 Problems
1.
Data for the LF356 op-amp (same as LF156) are given at the National Semiconductor
Web Site: http://www.national.com/ds/LF/LF155.pdf (there is a link from the 3330 web
page and a copy in the lab). Verify that the following values are reasonable
approximations to use in solving the problems. Identify your source of each item by
quoting the graph or table number and page number.
DC voltage gain
Unity gain frequency
Maximum output voltage
Maximum slew rate
Maximum output current
Input resistance
Output resistance
Input offset voltage
Input bias current
2.
A0 = 2·105 (same as AVOL in tables)
fT = 5 MHz (same as GBW in tables)
Vsat = ± 13 V (same as VO in tables)
SR=(dVout/dt)max = 12 Volts/µsec
(IO)max = 25 mA
Ri = 1012  (RIN in tables)
Ro = 40  (See the theory section for help with Ro.)
VOS = 3 mV
IB = 30pA
Calculate the values of low frequency gain G0 and the bandwidth fB for the non-inverting
amplifier in Fig. 4.2. for:
(a) RF = 50  and R = 50 
Gain: 2
Bandwidth: 2.5MHz
(b) RF = 50 k and R = 50 k
Gain: 2
Bandwidth: 2.5MHz
(c) RF = 50 k and R = 5 k
Gain: 11
Bandwidth: 450kHz
(d) RF = 0  and R = infinity (the voltage follower)Gain:1 BW: 5MHz
Be careful about which equations you use here. Make sure you justify when you can use
an approximate equation i.e., what you get from the golden rules for an ideal op-amp,
and when you need to do some extra work.
The point of the comment above is mostly regarding the concern that the Gain=2 amplifier of
part (a) might push the op-amp to produce its max current near the rail.
(e) Draw Bode plots (log-log; use the voltage gain instead of ‘dB’ for the y-axes for this
time) for the open loop gain and the closed loop gains from 2b, 2c, 2d above on the
same graph. What is the exponential rise-time for square waves in each case (see
page 4.5)? Make a table of your results.
3.
Estimate the input and output impedances of the amplifier with RF = 20 k and R =
150  for 1 kHz sine waves.
4.
What is the maximum output amplitude for 5 MHz sine waves if they are not to be
Physics 3330
Solutions 4.1
Physics 3330
Solutions Pre-lab 4
Spring 2013
distorted by the slew rate of the LF356 op-amp? (See H&H p. 192.) Draw a voltage vs.
time plot showing a sine wave input and a typical slew rate limited output on the same
plot. (Note that an amplifier limited by slew rate will change a curved line to a straight
line if the slope of the curved line would exceed the slew-rate.)
5.
Design an inverting amplifier (Fig. 4.3) with a closed-loop gain of 20. Select
appropriate values for R and RF to avoid the potential problems you found in question 2
above. Choose RF large enough so that the maximum output current of the op-amp is not
exceeded when the output is near the saturation voltage (~13V). Predict the bandwidth
and the input impedance at the signal input. How would you measure the input
impedance in the lab? [The DMM is not going to properly read the input or output
impedance when connected to a circuit because of all the other resistors and bias
voltages present. Here is a hint: Recall that when the signal generator with a 50 
output impedance has a 50  load the signal into the load drops by a factor of two.
Physics 3330
Solutions 4.2