ESE 211 / Spring 2011 / Lecture 10

Bipolar Junction Transistor

Let us first consider general transconductance amplifier loaded with short circuit

Transconductance

Obviously, power supplies are needed for circuit to operate. Often circuit requires bias and transconductance are defined for small variations of the input voltage near the bias point (input voltage Q-point). This small input voltage variation produces vartiation of the output current near its bias point (output current Q-point). Hence, defined in this manner the transconductance is small signal transconductance.

Once biased properly, the general transconductance amplifier can be replaced with its low frequency equivalent circuit for low frequency AC (signal) analysis.

Rin and Rout – are small signal input and output impedances of the transconductance amplifier at the given bias

Voltage controlled current ideal source 1 Net Transconductance from signal to load

Signal source has internal impedance and load is almost never can be considered short circuit.

Amplifier input voltage, i.e. voltage across Rin is smaller than signal voltage

Load current is smaller than internally generated current because of amplifier finite output impedance

Net transconductance from siggpnal to load is smaller then amplifier short circuit transconductance.

2 Transconductance amplifier output IV

v v in out G  g When Rin   , Rout    then m m iin iout

Iout

I Q2 out Vout vout Rout     Iout Q iout Q1 Iout Iout

Vout

Current independent on voltage can be obtained from pn-junction diode under reverse bias.

Reverse bias

Trying to move electron from p to n and holes from n to p, but there is very little number of electrons in p and holes in n!

3 Idea of BJT transistor

Reverse biased pn-jjppygunction can not supply large current. This current is fixed rather than controllable. Consider this: Forward biased pn-junction Reverse biased pn-junction

Can inject a lot of excess Current is small but independent on VRB electrons into p-section

Small number of electron is available

Output current remains independent on output voltage but becomes determined by input voltage/current 4 npn BJT in common base configuration

Useful current is current of electrons.

Common base current gain

Common emitter current gain

In properly designed transistor α ≈ 1 and β≈100. Useful current is current of holes This is true only in forward active regime, i.e. when base-emitter junction is forward biased and base- collector junction in not forward biased.

5 npn BJT in common emitter configuration

Input IV Output IV

0.7 V 0.3 V

Operation regimes:

VBE ≈ 0.7 V and VCE > 0.3 V - FORWARD ACTIVE

VBE ≈ 0.7 V and VCE < 0.3 V - SATURATION <

VBE < 0.7 V - CUTOFF 0

* base-emitter not forward biased and base-collector forward biased - REVERSE ACTIVE 6 Small signal parameters of BJT in forward active regime

Output IV Input IV Observe finite slope of output IV in real devices due to base width modulation (Early effect)

real ideal Early voltage

~ 0.7 V

0.3 V SllilttSmall signal output impedance Small signal transconductance

Small signal input impedance Small signal parameters relate linearly the variations of voltage and currents near their respective bias (Q) points 7 Small signal equivalent circuit of BJT in forward active regime

8 BJT Common Emitter Amplifier

Bias c ircu it to de fine: Q Q IC ,VCE , gm,r ,rO Q Q IC  I B  

Q VBB  0.7V I B  RB Q Q VCE  VCC  IC  RC Q Make sure that VCE  0.3V

Coupling cap – short Than calculate small signal for signal, open for DC. parameters Q IC β VA gm  ,rπ  ,rO  Q Vth gm IC Open circuit signal voltage gain. And use small signal equivalent circuit for signal analysis

* The bias scheme shown in this circuit has serious problems. It is used here for simplicity. Realistic stable bias scheme will be considered later. 9 BJT Common Emitter Amplifier

Assume base bias current of 10 uA, ex. RB = 100k and VBB = 1.7 V. Common emitter current gain of about 100 and Early voltage about 100 V Often can be neglected

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