# G2-1: Transistor Characteristics¶

## Apparatus¶

$$3\text{V}$$ battery; $$9\text{V}$$ battery; 2 rheostats (high resistance); resistor $$\text{R}_2$$ (approx. $$50\text{k}\Omega$$); voltmeter ($$0-5\text{Vdc}$$); ammeter ($$\approx 50 \mu\text{A}$$ fsd); ammeter ($$\approx 3 \text{mA}$$ fsd); transistor (pnp); connecting leads (12 short); 2 sheets graph paper.

## Instructions¶

Set up the circuit as above, but do not connect the batteries until a teacher has checked the circuit (to avoid damaging the ammeters or transistor). In the experiment, when not taking readings, leave the batteries disconnected.

## Experiment 1¶

To investigate the ‘transfer characteristics’ of the transistor. The transistor acts as a current amplifier: the size of the large current $$I_C$$ depends on the size of the small current $$I_B$$. The circuit used above is called a ‘common emitter’ circuit.

### 1: Procedure¶

1. Set $$V_{CE}$$ to $$4\text{V}$$ using rheostat $$\text{R}_3$$. Ensure that this remains constant (adjust $$\text{R}_3$$ again later as necessary).
2. Set $$I_B$$ to 0 using $$\text{R}_1$$. Read and note $$I_B$$ and $$I_C$$.
3. Increase $$I_B$$ a little using $$\text{R}_1$$, and read and note $$I_B$$ and $$I_C$$. Continue increasing $$I_B$$ and reading the ammeters until $$I_C =$$ 3 mA.
4. Tabulate the readings of $$I_E$$, $$I_C$$, and the value of $$V_{CE}$$.

### 1: Analysis¶

1. Plot a graph of $$I_C$$ against $$I_B$$, labelling the curve with the value of $$V_{CE}$$ used.

2. Find the gradient of the straight-line section of the curve. Then:

$\text{Current gain } \beta = \frac{\Delta I_C}{\Delta I_B} = \text{gradient}$

## Experiment 2¶

To study how $$I_C$$ varies when $$V_{CE}$$ is changed, for certain fixed values of $$I_B$$.  The graph obtained is called the ’output characteristic’ of the transistor.

### 2: Procedure¶

1. Set $$I_B = 0$$ using $$\text{R}_1$$. Starting with $$V_{CE} = 0$$, and little by little increasing $$V_{CE}$$ up to $$5\text{V}$$, take a set of readings of $$I_C$$ and $$V_{CE}$$ and note the value of $$I_B = 0$$.
2. Increase $$I_B$$ to $$10 \mu$$A, and obtain another set of readings of $$I_C$$ and $$V_{CE}$$ as in step 1.
3. Repeat the procedure with $$I_B = 20 \mu$$A then $$30 \mu$$A.
4. Tabulate the sets of readings of $$I_C$$ and $$V_{CE}$$, noting the value of $$I_B$$ for each set.

### 2: Analysis¶

1. Plot a graph of $$I_C$$ vs. $$V_{CE}$$ to obtain four curves. Label each curve with the appropriate value of $$I_B$$ used.

## Questions¶

1. When $$I_B = 0$$, $$I_C$$ should be zero for all $$V_{CE}$$. However all transistors have some leakage current. What is the value of the leakage current $$I_C$$ when $$V_{CE} = 4$$V?

2. What is the approximate minimum $$V_{CE}$$ so that a variation in $$I_B$$ between $$0$$ and $$30 \mu A$$ produces a large change in $$I_C$$? (In practice the supply voltage is usually set between this value and a certain maximum. The maximum depends on the ‘breakdown voltage’ of the junctions).

3. In use as an amplifier, an AC input voltage makes $$I_B$$ vary with time. For example:

1. Use the value of $$\beta$$ to make a graph of $$I_C$$ against time.
2. If a resistor $$\text{R} = 1$$k$$\Omega$$ is connected in
series with the collector $$C$$, so that $$I_C$$ flows through it; draw a graph of the potential difference (p.d.) across this resistor against time.
3. What is the frequency of these AC currents and p.d.?
4. Draw a diagram to show while the pnp transistor is conducting:

1. Electron flows and conventional currents through the three terminals.
2. Electron & hole movements inside the transistor (may be simplified).