(a) Draw the circuit arrangement for studying the input and output characteristics of an n-p-n transistor in CE configuration . With the help of these characteristics define (i) input resistance , (ii) current amplification factor .
(b) Describe briefly with the help of a circuit diagram how an n-p-n transistor is used to produce self-sustained oscillations .

Explanation

(a) Common emitter (CE) transistor characteristic : The transistor is most widely used in the CE configuration . When a transistor is used in CE configuration , the input is between the base and emitter and the output is between the collector and emitter .
The input and output characteristics of an n-p-n transistor in CE configuration can be studied by using the given circuit .

(i) Input characteristics : The variation of the base current `I_(B)` with the base emitter voltage `V_(BE)` is called the input characteristics keeping `V_(CE)` fixed . A curve is plotted between the base current `I_(B)` against the base emitter voltage `V_(CE)` is kept fixed .
Since `V_(CE) = V_(CB) + V_(BE)` and for Silicon (Si) transistor `V_(BE)` is 0.6 to 0.7 `V , V_(CE)` must be larger than 0.7 V . The input characteristics of a transistor is shown in fig. (a) .
(ii) Output characteristics : The variations of the collector current `I_(C)` with the collector emitter voltage `V_(CE)` , keeping the base current `I_(B)` constant is called output characteristics .
The plot of `I_(C)` versus `V_(CE)` for different fixed values of `I_(B)` gives one output characteristic . The different output characteristics for different values of `I_(B)` is shown in ltbegt

(iii) Input Resistance : This is defined as the ratio of the change in collector-emitter voltage `(DeltaV_(CE))` to the resulting change in base current `(Delta I_(B))` at constant collector-emitter voltage `(V_(CE))`.
`therefore " " r_(i) = ((DeltaV_(BE))/(DeltaI_(B)))_(V_(CE))`
(iv) Output Resistance : This is defined as the ratio of the change in base-emitter voltage `(Delta_(VE))` to the change in collector current `(DeltaI_(C))` at constant base current `I_(B)`.
`therefore " " r_(0) = ((DeltaV_(CE))/(DeltaI_(C)))_(I_(B))`
(v) Current Amplification Factor `(beta)`: This is defined as the ratio of the change in collector current `(DeltaI_(C))` to change in base current `(Delta I_(B))` at constant `V_(CB)`.
`beta_(ac) = ((DeltaI_(C))/(DeltaI_(B)))_(V_(CE))`
This is also known as current gain .