Draw a circuit diagram of a transistor amplifier in CE configuration. Define the terms: (i) Input resistance and (ii) Current amplification factor. How are these determined using typical input and output characteristics?

The input resistance is defined as the ratio of the input voltage to the input current. The input voltage is Vin and the input current is simply the differential input current Id. Input Resistance = Vin / Id

Current Amplification Factor or Current Gain is basically the ratio of the output current to the input current when it's passing through an electrical device. So, it gives us the amount of current gained as it flows through. Current Gain (β or hfe) = Ic / Ib, where Ic is the collector current and Ib is the base current.

Determining these using input and output characteristics:

• Input Resistance: The input characteristic curves plot Ib (base current) against Vbe (base-emitter voltage). The input resistance at a particular operating point can be determined by finding the slope of the tangent to the curve at that point. ΔVbe / ΔIb represents the incremental input resistance at that operating point.

• Current Amplification Factor: The output characteristic curves plot Ic (collector current) against Vce (collector-emitter voltage) for different values of Ib (base current). To find the current gain (β) at a specific operating point, select a point on the output characteristic curve corresponding to a chosen value of Ib. Then, find the value of Ic at that point. The current gain (β) at this point is simply Ic / Ib.

Note: These methods provide the incremental or dynamic input resistance and current gain, which are relevant for small signal operation. The static values (calculated using DC bias conditions alone) can vary significantly from the dynamic values.