Chemistry 498A Laboratory, Spring 1990

## Transistor Switches and Amplifiers

Equipment required: Hitachi Model V-355 dual-trace 35 MHz oscilloscope, EICO sine-square wave signal generator, digital multimeter, 0-20 volt variable power supply, Breadboard; 2N2030 NPN silicon transistor (or similar), DC relay with 6-28 volt coil, 25 µf, 15 volt (or greater) electrolytic capacitor, 20 KOhm potentiometer, resistance substitution box, 10 KOhm and 1 KOhm, 10%, 1/2 watt resistors.

A. Basic characteristics of relays
Use the resistance ranges of your digital multimeter to locate the coil connections on the relay supplied by the instructor and to measure its resistance. Connect the relay coil to your variable power supply and slowly turn up the voltage until you can hear and/or see the relay trip. Record this voltage and calculate the current required to trip (turn on) the relay.

B. Transistor switch
The objective of this part of the experiment is to design a transistor switch that will allow the relay to be operated from a 3 volt signal -- that is, relay off when the signal voltage is zero and relay on when the signal voltage is 3 volts. You will use a 2N2030 transistor, a general-purpose, medium power switch and amplifier transistor, which has the following characteristics:

Type NPN silicon
Current gain (ß ) 50 minimum
Maximum collector-emitter voltage 40 volts
Maximum collector current 700 mA
Maximum power dissipation 5 watts
Maximum frequency 100 MHz.

where Vcc is obtained from your variable power supply set to the nominal relay coil voltage, or if you don't know that, to a voltage about 20% greater than the minimum voltage needed to trip the relay. Obtain the input voltage Ein from the Voltage Reference Source. Calculate the value of R1 so that when Ein is 3 volts, the collector current (and therefore the relay coil current) equals the current required by the relay coil, assuming the minimum current gain (ß) of 50. Test the circuit and verify that the relay trips when Ein is 3 volts.

Turn down the input voltage Ein until the collector voltage is 1/2 of Vcc. Measure the voltages across R1 and across Rc, calculate the currents through these resistors, and calculate the current gain, ß . _______________

Measure the collector-emitter voltage when the transistor is conducting, that is, when Ein is 3 volts. __________________ volts.

Question: What would happen to the collector current if the ß of the transistor were larger than 50 or if the input voltage were larger than 3 volts (say, 5 volts)? Why?________________________________________

________________________________________________________

C. Transistor AC amplifier
1. Set up the following circuit on your breadboard, where Rc is 10 KOhm and Re is 1 KOhm (both 10%, 1/2 watt). Obtain the +12 volts from the 12 volt power supply that is built in to the breadboard, and the input voltage from the sine-wave output of your signal generator, Set the frequency to 1000 Hz and turn the output level all the way down -- counterclockwise -- to start with. Pay attention to the polarity of the 25 µ f electrolytic capacitor. Use the box-mounted ten-turn Helipot for the 20 KOhm pot and set it all the way down -- counterclockwise -- to start with. Observe the output voltage Eout on the oscilloscope.

2. Slowly turn up the 20 KOhm pot until the collector voltage with respect to ground is 1/2 of the 12 volt power supply voltage. Now, turn up the sine wave level control on the signal generator carefully and observe the scope trace. Adjust the input level until the output signal at the collector is at least 6 volts peak-to-peak but not so large that it clips. Adjust the 20KOhm pot so that the output waveform clips evenly, and then turn down the input signal level until the output waveform is no longer clipped.

Measure the AC voltage gain of this amplifier (the ratio of the AC output voltage to the AC input voltage). ____________________________

Note: It should be equal to the ratio of Rc to Re (that is, about 10).