What are the two main characteristics of a transistor?

Answer 1

- The ratio of the dc collector current (IC) to the dc base current (IB) is thedc beta (bDC).

- bDC is called the gainof a transistor:

bDC = IC/IB

- Typical values of bDC range from less than 20 to 200 or higher.

- bDC is usually designated as an equivalent hybrid (h)parameter:

hFE = bDC

- The ratio of the collector current (IC) to the dc emitter current (IE) is the dc alpha (aDC). This is a less-used parameter than beta.

aDC = IC/IE

- Typical values range from 0.95 to 0.99 or greater.

- aDC is always less than 1.

- This is because IC is always slightly less than IE by the amount of IB.

- From graph above we can see that there are 6 important parameters to be considered:

i) IB: dc base current.

ii) IE: dc emitter current.

iii) IC: dc collector current.

iv) VBE: dc voltage at base with respect to emitter.

v) VCB: dc voltage at collector with respect to base.

vi) VCE: dc voltage at collector with respect to emitter.

- VBB forward-biases the BE junction.

- VCC reverse-biases the BC junction.

- When the BE junction is forward biased, it is like a forward biased diode:

VBE ? 0.7 V

- But it can be as high as 0.9 V (and is dependent on current). We will use 0.7 V from now on.

- Emitter is at ground. Thus the voltage across RB is

VR(B) = VBB- VBE

- Also

VR(B) = I­RRB

- Or:

I­RRB = VBB- VBE

- Solving:

IB = (VBB- VBE)/RB

- Voltage at collector with respect to grounded emitter is:

VCE = VCC - VR(C)

- Since drop across RC is VR(C) = ICRC the voltage at the collector is also:

VCE = VCC - ICRC

- Where IC = bDCIB. Voltage across the reverse-biased collector-bias junction is

VCB = VCE - VBE

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