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Collector Characteristic Curves

Consider the circuit of Figure 96, which we will use to learn about the operation of the transistor in a circuit by adjusting the voltage sources V_BB and V_CC.

  

Figure 96: Basic transistor circuit.

A graph of the collector characteristic, i_C versus v_CE is shown in Figure 97.

  

Figure 97: Collector characteristic curves

Suppose V_BB and V_CE are set to produce a current i_B > 0 and V_CE=0 V. Then both BE and BC junctions are forward biased. The saturation region   corresponds to the case where both junctions are forward biased.

As v_CE increases and reaches about 0.7 V, the BC junction becomes reverse biased (before this, the transistor is in saturation). When v_CE exceeds 0.7 V, the transistor goes into the active region,   or linear region   corresponding to the BE junction forward biased and BC reverse biased. This is the normal mode of operation of the transistor, e.g. as in amplifiers. In the active region the collector characteristic is almost flat, meaning that i_C is almost constant but v_CE can vary. The value of i_C is determined by i_B,

$$i_C = \beta i_B .$$ (84)

If v_CE increases too much, the transistor goes into the breakdown region. This should be avoided.

Figure 98 shows collector curves for different values of base current. When i_B=0 A, the transistor is in its cutoff region  , and only a small leakage current flows as i_C.

  

Figure 98: Collector characteristic curves for various values of I_B.

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