21 BJT Circuits, Gain and Design

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Transcript of 21 BJT Circuits, Gain and Design

BJT Circuit Configurations

~V be

vs Rs

RL V cc


vs Rs

RL V cc


vs Rs

RL V cc

Common base

Common emitter

Common collector

Common emitter current gain

BJT Current-Voltage Characteristics

VCE, V Very small base current (~5-75 A) causes much higher collector current (up to 7.5 mA). The current gain is ~ 100

BJT amplifier circuit analysis: Operating point



Collector current depends on two circuit parameters: the base current and the collector voltage. At high collector voltage the collector current depends on the base current only. For Ibase = 40 A, Icoll = 4 mA for any EC-E greater than 1.5 V

BJT amplifier circuit analysis: Operating point VCC



For an arbitrary collector voltage, collector current can be found using the KVL. The KVL for the collector emitter circuit, VCC = IRLRL + VCE;

V V I RL = CC CE The RL current depends linearly on the collector voltage VCE RLResistor RL and the C-E circuit of BJT are connected in series, hence IRL = IC For Ib = 40 A and VCC = 13V, the collector current IC = 4 mA For Ib = 75 A and VCC = 14V, the collector current IC = 4 mA

BJT amplifier gain analysis: 1

1. Input circuit The input voltage has two components: the DC bias and the AC signal Vin AC signal amplitude DC bias Time

DC voltage component biases the base-emitter p-n junction in the forward direction AC component is the input signal to be amplified by the BJT.

BJT amplifier gain analysis: 2


2. Output circuit The collector current has two components too. IC AC current amplitude Base current DC current DC and AC collector currents flow through the BJT in accordance with its I-V characteristics


BJT amplifier gain analysis: 3

The resistance of the B-E junction is very low when VBE VBE0 Vbi 0.7 V. Hence the base current IB (Vin-VBE0)/R1 = (VinDC-VBE0+VinAC)/R1 The collector current IC does not depend on the collector voltage if the latter is high enough. Hence, IC IB; The voltage drop across the load resistance R2: V2= IC R2; The output voltage Vout = VCC - V2 = VCC - IC R2; Vout = VCC - IC R2 = VCC - IB R2 = VCC - R2(VinDC-VBE0+VinAC)/R1; In signal amplifiers only AC component of the output voltage is important: VoutAC = - R2VinAC/R1; The amplifier voltage gain: kV


= VoutAC/VinAC= - R2/R1;

BJT amplifier gain analysis: 4


Common emitter gain summary: Current gain: kI = Voltage gain: kV

= VoutAC/VinAC= - R2/R1; R2/R1

Power gain: kP = kV kI = - 2

BJT design and factors affecting the performance

Base resistance and emitter current crowding in BJTsThe voltage drop along the base layer Vbb = rbb Ib,where rbb is called the base spreading resistance. The p-n junction current decreases rapidly when the voltage drops by Vbb ~ Vth = kT/q de

VV qV kT I = I S e 1 = I S e TH 1 The length of the edge region, de, where most of the emitter current flows may be estimated from:

Vth = I b Rb max

d e I b b te W

Emitter current crowding in BJTs (cont.)

Vth = I b Rb max

d e I b b te W


1 b = qN b b

From these,

Vth te W Vth q N b b te Wb d e = Ib b Ib


Estimate the effective emitter length, de for the BJT having the following parameters:Ib = 50 A; Nb = 1017 cm-3de

b = 400 cm2/V-s Wb = 0.1 m te = 100 m

Vth q N b b te Wb d e Ibde = 3.33 e-4 cm = 3.33 m

Large periphery B JT Design

Base narrowing (Early effect)The depletion width of the p-n junction depends on the applied voltage:

(Here W is the depletion region width not the width of the base as in BJT!)

In the BJT, this effect means that the effective width of the base is less than Wb: Weff = Wb - Xdeb - Xdcb where Wb is the physical thickness of the base, xdeb and xdcb are the depletion region widths from the emitter and collector sides. The depletion widths are given by (Ne>>Nb):

2 0 (Vbi Vbe ) xdeb = q Nb

1/ 2

2 0 (Vbi + Vcb ) N c x dcb = 2 qN b high.

1/ 2

xdcb is usually the most important factor since the voltage applied to collector is The doping level in collector, NDC has to be lower than that of the base, NAB, to reduce the Early effect: