Electronics

PNP junction transistor

The PNP junction transistor pictorial presentation is:

V_EE forward-biases emitter-base pn junction, electrons in base flow to emitter. Emitter base junction property is low impedance and low voltage drop but collector base junction is reverse-biased with high impedance. Current in the emitter circuit is emitter current while current in the collector circuit is collector current. Collector current comprised of emitter current and collector base current, I_CBO. The emitter current has constant of proportionality α or αI_E.

Both currents αI_E and I_CBO . This is written as I_C =  αI_E + I_CBO . Apply KCL to this transistor yields

I_E = I_B + I_C

β

From I_E = I_B + I_C , I_B = I_E - I_C. But  I_C =  αI_E + I_CBO rearrange α we get

The ratio α/(1-α) represents β.

Emitter Base Junction

Apply KVL to emitter base loop the emitter current is I_E = (V_EE - V_EB)/ R_e . Simplify circuit that voltage collector dependence is negligible and dependence of emitter current on collector base voltage is postponed then emitter-base circuit is exactly same as the analysis of diode circuits.

Voltage threshold or V_EBQ is about 0.7 for silicon transistor. When VEE = 4 volts and Re = 1 kΩ. Note that V_BE varies between 0.7 and 0.9 volts depends on base current.

DC with oscillator

Insert oscillator v_i in series with dc source V_EE. The ac signal is v_i = V_im cosωt . The presentation picture is

Figure One

Collector Base Junction

Collector Base junction is bring into consideration after plotting the common-base output characteristics. Such a  figure consider below:

For values V_CB less than 0.5 volts the curve plotted above is a family of straight line equation given by

I_C = αI_E + I_CBO

Convert this equation into its equivalent yields:

is

Active elements like transistor has emitter current transmitted to collector current thus αI_E is a dependant source. To avoid the nonlinear region to the left of i_C axis it is necessary that V_CB less than 0.5 volts at all times.

Large signal model

Figure two

In the circuit above α = 1, I_CBO = 0, VEE = 2 volts, Re = 1 kΩ, V_CC = 50 volts, R_C = 20 kΩ and a 1 volt peak sinusoidal source is connected in series with V_EE.

For emitter-base circuit, the junction is forward biased as long as V_im < 1.3 volts. The transistor amplifies input ac voltage and resulting voltage gain is A_O = V_cbm/V_im = 20/1 = 20.

Current amplification

A change in emitter current in pnp transistor produces a change of approximately the same amount of collector current iC and smaller change in base current a factor of 1 - α. To achieve current amplification the change is initiated in the base current not in the emitter current. It assumes that β = α/(1-α) does not vary with base current. Neglecting I_CBO, we have i_C = βi_B . In a typical range of 1 to 100 mA, h_fe is approximately equal to β, independent of any changes in collector current.

Current amplification circuit

Current amplification circuit is made so that base-emitter junction is forward-biased and collector-base junction reverse-biased. The picture presentation is

The derivation involved in this circuit is made based on Kirchoff's Voltage Law

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