E-10: 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

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 $I_{c}$ depends on the base current $I_{b}$. We write I $_{c} = \beta I_{b}$ where $\beta$ is the current amplification factor.

SUGGESTED EXPERIMENTS:

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

Figure 2

Start with the voltage divider completely counter-clockwise so that the emitter-base voltage $V_{eb}$ is a minimum. With the variable power supply set to 3 V, record the $I_{b}$ measured on DMM$_1$ and the $I_{c}$ on DMM$_2$

Repeat the readings for ten reasonably spaced (higher) settings of the voltage divider (i.e. higher $V_{eb}$ and hence higher $I_{b}$.

Calculate $\beta (=I_{c}/I_{b}$) and plot it against $I_{b}$. Over what range of I$_{b}$ is $\beta$ reasonably constant? Repeat the above but with emitter to collector voltage $V_{ec}$ now at 7 V.

2.

(Optional) MEASUREMENT OF $I_{c}$ vs $V_{eb}$: Remove the multimeter DMM$_1$ (used as a microammeter) from the circuit of Fig. 2 and use it instead as a voltmeter to measure the emitter to base voltage $V_{eb}$. With the variable power supply set to give 7 V for $V_{ec}$, use the voltage divider to vary $V_{eb}$. Read and record both $V_{eb}$ and $I_{c}$ for 10 reasonably spaced values of $V_{eb}$. Plot $I_{c}$ vs $V_{eb}$. The results are very similar to a diode curve (as expected since $I_{c}$ is proportional to $I_{b}$ 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

The input voltage to the transistor $V_{in}$ includes the drop across the $4.7 \mbox{k}\Omega$ protective resistor. The voltage gain is $\mbox{G}=\Delta V_{out}/\Delta V_{in}$. Record input voltages $V_{in}$ and output voltages $V_{\mbox{\tiny out}}(=V_{ec}$) for ten reasonably spaced values of $V_{in}$ from 0 to 5 volts.

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

4.

Distortion effects when amplifier is overdriven: Set up the circuit plug board as in Fig. 4. Connect Y$_1$ and Y$_2$ 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

Adjust the voltage divider until the $V_{in}$ 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 Y$_2$.

Observe and record the effect of changing V$_{in}$ to values outside of the operating range (where $\beta$ is constant).