E-8: Transistors

OBJECTIVE:

To experiment with a transistor and demonstrate its basic operation.

APPARATUS:

An npn power transistor; dual trace oscilloscope & manual; signal generator & frequency counter; power supply ($ \pm$15 V fixed/$ \pm$9 V variable); circuit plug board & component kit; two digital multimeters (DMM); differential amplifiers.

INTRODUCTION:

Our junction transistor (Fig. 1) is like two back to back np diodes (see E-8 Part C, #3). Hence there are two possibilities, an npn transistor and a pnp transistor. Our npn transistor has a central p-type layer (the Base) between two n-type layers (the Emitter and Collector). There are other type transistors which we will not discuss, (e.g. a MOSFET: Metal-Oxide Semiconductor-Field-Effect-Transistor).

Figure 1
\includegraphics[height=2.in]{figs/e10-01.eps}

For an npn transistor the collector is positive relative to the emitter. The base-emitter circuit acts like a diode and is normally conducting (i.e. forward-biased). The base-collector circuit also acts as a diode but is normally non-conducting (reverse biased) if no current flows in the base-emitter circuit. However when current flows in the base-emitter circuit, the high concentration gradient of carriers in the very thin base gives an appreciable diffusion current to the reversed biased collector. The resulting collector current Ic depends on the base current Ib. We write I c = $ \beta$Ib where $ \beta$ is the current amplification factor.

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Although our powe...
...; read currents and voltages to two significant figures only.
\smallskip \par
}}

SUGGESTED EXPERIMENTS:

1. CURRENT Amplification: Measure $ \beta$ for various emitter to collector voltages, Vec. Set up the circuit as in Fig. 2.

Figure 2 \includegraphics[height=1.8in]{figs/e10-02.eps}

Start with the voltage divider completely counter-clockwise so that the emitter-base voltage Veb is a minimum. With the variable power supply set to 3 V, record the Ib measured on DMM1 and the Ic on DMM2

Repeat the readings for ten reasonably spaced (higher) settings of the voltage divider (i.e. higher Veb and hence higher Ib.

Calculate $ \beta$(= Ic/Ib) and plot it against Ib. Over what range of Ib is $ \beta$ reasonably constant? Repeat the above but with emitter to collector voltage Vec now at 7 V.

2.
(Optional) MEASUREMENT OF Ic vs Veb: Remove the multimeter DMM1 (used as a microammeter) from the circuit of Fig. 2 and use it instead as a voltmeter to measure the emitter to base voltage Veb. With the variable power supply set to give 7 V for Vec, use the voltage divider to vary Veb. Read and record both Veb and Ic for 10 reasonably spaced values of Veb. Plot Ic vs Veb. The results are very similar to a diode curve (as expected since Ic is proportional to Ib over the region where $ \beta$ is a constant).
3.
VOLTAGE Amplification: By adding a large load resistor in series with the collector, one can convert the current amplification $ \beta$ observed earlier into a voltage amplification. Hook up the circuit plug board as in Fig. 3.

Figure 3
\includegraphics[height=3.in]{figs/e10-03.eps}
The input voltage to the transistor Vin includes the drop across the 4.7 k$ \Omega$ protective resistor. The voltage gain is G = $ \Delta$Vout/$ \Delta$Vin. Record input voltages Vin and output voltages Vout(= Vec) for ten reasonably spaced values of Vin from 0 to 5 volts.

Graph Vout vs Vin and calculate the voltage gain G at the steeply changing part of your graph. The value of Vin at the center of this region we call the ``operating voltage'' of the amplifier. How would the gain change if the load resistance was 2.2 k$ \Omega$ instead of 10 k$ \Omega$?

4.
Distortion effects when amplifier is overdriven: Set up the circuit plug board as in Fig. 4. Connect Y1 and Y2 to the scope. To connect the signal generator to the board, use the BNC to banana plug adapter and remember that the side with the bump goes to ground.

Figure 4
\includegraphics[height=3.2in]{figs/e10-04.eps}

Adjust the voltage divider until the Vin is close to the ``operating voltage'' found in #3.

Vary (and record) the amplitude of the input signal from small values to those which overdrive the amplifier and produce considerable distortion in the output Y2.

Observe and record the effect of changing Vin to values outside of the operating range (where $ \beta$ is constant).


Michael Winokur 2007-09-07