Lab 1: Gain-Bandwidth Product in LF353 op amp

Background:  After the transistor, the op amp is probably the next most important analog circuit building block. In EN123 we'll see op amps in the service of frequency filters, so-called active filters. Frequency specification is important for any op amp, and in this lab we will investigate the LF353 op amp's frequency response by measuring its gain-bandwidth product (GBWP). Basically, the actual gain of an op amp circuit times the frequency of its input signal must be less than the op amp's GBWP.

http://www.national.com/pf/LF/LF353.html

Requirements:
(1) Build an op amp circuit with a gain of -10. Test your circuit by showing that an input of 0.1v results in an output of -1.0v. Be sure to activate the Track feature of the triple-output power supply for your +/- 15v needed at pins 8 and 4 on the 353...

(2) Use the 54622A oscilloscope to observe from the Agilent 33120A waveform generator a 10KHz sine wave with a peak-to-peak signal of 0.5v.

(3) Show that when the sinewave is used as the input to your gain of -10 circuit, the output is a 5v p-p sinewave, gain of -10. Watch both the input and output to the circuit to see that the waveform is inverted.

Warning: Do not connect the output of the 33120A directly to a power supply output or the 33120A output may be damaged, or an output fuse may blow. The output fuse is hardwired to a PC board inside the box, and is a... a difficult part to replace.

Note on Menu D of the 33120A you can change the termination from 50 Ohms to High Z, so the peak to peak reading from the instrument agrees with the value seen on the scope.

(4) Now gradually increase the input frequency until the gain falls to a value of -5. What is the frequency? What does that imply the GBWP of your circuit is? Actually, calculating the GBWP depends on the frequency of the input sinewave, so we should say that we would like to know the frequency at which the gain drops to 1 (from a much higher value of the gain setting resistors). What does the data sheet say the LF353 GBWP should be?

(5) Repeat the same experiment with a wired gain of -100 and a p-p input of 50 mV. To have a 50mV p-p output you will need to select 50 ohm termination, and use a 51 ohm resistor to ground where the sinewave goes into your op amp input. Is the GBWP product the same as part 4?

(6) Now build an op amp circuit (one op amp only) with a gain of +10 (input to the +V pin) and reassess the GBWP. Measure also for a gain of +100. Your positive gain op amp will be governed by

as seen in lecture and in the online notes (link below).

Put your GBWP answers in the form of a table, in Word, in your student work folder. Be prepared to demonstrate any of the GBWP experiments above.

(7) Find an electret microphone and its data sheet (such as DigiKey part 359-1007, from Horn Industrial Co.) and use it as an input to a high Zin positive gain op amp. Some mic's have been wired: orange for signal and green for ground. The signal connection must go through a 500 - 1000Ω resistor to a 1.5-2v supply.

link to site discussing sound level measurements in decibels

Whistle into the (1.5v powered) electret microphone and observe the output of the op amp on your 54622A 'scope. Increase the gain of the op amp until you can produce an oscillation on the scope greater than 1v p-p. (whistling or singing, not hand clapping...) You will likely need to eliminate the 1 volt or so that is a DC component of the signal in order to amplify the oscillating component without saturating the output. Try a C-R high pass filter (0.3 μF and 1MΩ) or try subtracting 1.2 volts with a negative gain summation amplifier.

Do Lab 1 with your partner.

Possible FTQs: (1) With a particular input frequency in the range 100K to 1MHz, and your op amp wired for gain of -100, what will be the output amplitude?
(2) What happens to gain if R1 is pulled out of the positive gain op amp?

Free Advice: Find the pinout of the LF353 from the National Semiconductor website. www.national.com.
Note which is pin 1 (bottom left pin when you can read the print on the chip). Push the chip pins into holes across the channel on a piece of white solderless breadboard. You'll need to use the Tracking feature of your triple output power supply to provide ± 12-15 volt power to the chip. You may want to review the first few pages of Lab 1 (2001) on the EN123 archive website.

Also look at Lecture notes on Amplifiers from the homepage, or see the archive lecture notes from 1999.

See below for snapshot from Horn part 359-1007 pdf datasheet, found through digikey.com:
You can let your capacitor be 0.3 muF, from the reel near the emergency shower in 095.

If you haven't used an oscilloscope before, ask JD or the TA for a brief introduction.

Possible FTQ: Show how, using two diodes and 4 op amps, you can make a full wave rectifier for a sinusoid input. In other words, an absolute value circuit... hint: see amplifer notes about