As shown in above circuit transistor consist of two PN junctions, so that the transistor can be thought of as two diodes- in this case back-to-back position. Diode point outward as arrow on the emitter lead of the NPN transistor.
In normal amplifier, as shown in above circuit the base-emitter junction is always forward bias by VBB and collector-base junction is always reverse bias by Vcc. Moreover, since VCE=VCB+VBE and VBE is very small then VCB=VCE
This diagram is used to emphasize the correct biasing of the transistor.
Transistor Input Characteristics:
As shown in above diagram two diode analogy is used to explain the input characteristic of a transistor. Since base-emitter junction is forward bias, VBE is very small and 0.6V for germanium transistor and 0.2V for silicon transistor.
The iB—–>VBE curve gives input characteristics for different values of output voltage VCE of transistor. From this graph we can say, as voltage between base and emitter increase (for the voltage greater than 0.6V for germanium) the base current iB increases sharply for different values of output voltage VCE. So what happen at the input depends upon the output voltage VCE We get different curve for different values of output voltage VCE. This difference is due to change in depletion region under reverse bias condition.