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FOUNDATION EXP 8 – BIPOLAR TRANSISTOR EXPERIMENT 8 BIPOLAR TRANSISTOR 1.0 INTRODUCTION A transistor is basically a three terminal semiconductor device that may be used to provide a number of functions. Its chief applications are use as a switch or as an amplifier. The three terminals of a bipolar transistor are labelled base, emitter and collector (see Figure 1.1). A small change of current in the base can give rise to a large change in the current flowing between the collector and emitter. Figure 1.1 The circuit diagram for a transistor is shown in Figure 1.2. This is of an NPN transistor, which will be used in this experiment. The diagram for a PNP transistor is similar, but the arrow on the emitter is pointing in the opposite direction. Collector Base Emitter Figure 1.2 2.0 TESTS 2.1 OPERATION AS A SWITCH A transistor’s function as a switch is the simplest mode of operation and involves turning the transistor from a fully on to a fully off state, or vice-versa. A simple switch is shown in Figure 2.1. When a sufficient positive voltage is Dr. Daniel Nankoo 1 of 4 FOUNDATION EXP 8 – BIPOLAR TRANSISTOR applied to the base (Vin) the transistor switches on. This allows a current to flow from the collector and emitter and hence Vout falls to a low value. +5V 1kΩ B Vout A Vin 10kΩ Figure 2.1 Connect up the circuit shown in Figure 2.1 on your breadboard. Take care to identify the base, collector and emitter correctly (see Figure 1.1). Connect point A to 0V and measure the value of Vout (voltage between point B and 0V). Connect point A to 5V and measure the value of Vout. Record your results in tabular form. Measurements should be taken with respect to ground. Also vary Vin and measure the value that makes Vout = 2.5V (i.e. half way between ON and OFF). 2.2 OPERATION AS AN AMPLIFIER VCC = +10v A RA 4k7Ω 10kΩ RC -10μF+ +10μF- 1kΩ INPUT 10kΩ POT RB B OUTPUT RE 120Ω C + CE = 47μF - 0V Figure 2.2 In this mode of operation, the transistor is set so that its output is halfway between the fully on and fully off values when no input signal is applied. This Dr. Daniel Nankoo 2 of 4 FOUNDATION EXP 8 – BIPOLAR TRANSISTOR is known as biasing. A small bipolar signal may then be added to the bias voltage on the base, to produce a larger (or amplified) output on the collector. A typical transistor amplifier is shown in Figure 2.2. Note the use of capacitors on the input and output to block the DC bias voltage which allows the varying (alternating) signal to pass (refer to Experiment 4). Connect up the circuit shown in Figure 2.2 on your breadboard. Choose RC to be 1k. Take care to identify the base, collector and emitter correctly and the polarity of the capacitors. Set the variable power supply to 10V DC and adjust the potentiometer (RB) to obtain a 5V DC level at the collector. Do not confuse the 10V power supply with the input voltage. The power supply is there to make the circuit work, whereas the input voltage, Vin, is the signal to be applied to the circuit. You will need to measure the voltage at the collector at the end of RC as the capacitor will block this bias voltage from reaching the output. If you are unclear on the connection or operation of the potentiometer, ask for help. A potentiometer has three legs. The leg in the middle represents the line with the arrow in the symbol for a potentiometer (or variable resistor) in the circuit diagram. Apply a 300mV peak-to-peak, 1kHz sinusoidal input signal (from the function generator) to the input of the amplifier. Use Channel 1 of the oscilloscope to set and display this signal. Connect Channel 2 of the oscilloscope across the output to display and measure the output signal. It is important to check that the output signal is nearly sinusoidal with no flattening around its peaks, so ask staff to double check your waveform. Make careful, scaled drawings of the Vin and Vout waveforms using the same time axis. From your measurements, calculate the voltage gain (AV) of your amplifier, using the following formula: AV = Vout / Vin Disconnect the power supply and carefully remove the lead connecting the wiper (middle leg) of the potentiometer to the base of the transistor. Use the DMM, set to measure Ohms, to measure the resistance RA between points A and B and RB, between points B and C. Record your results in your lab book in tabular form. Change the value of RC from 1k to 470 and repeat the above experiment, recording your results (Vin, Vout, AV, RA and RB) in your lab book in tabular form. Dr. Daniel Nankoo 3 of 4 FOUNDATION EXP 8 – BIPOLAR TRANSISTOR 3.0 COMMENTS AND CONCLUSIONS Comment on the effect of changing RC, and how you would go about designing an amplifier to achieve a certain gain AV. Dr. Daniel Nankoo 4 of 4