BJT Circuit Configurations

Transcription

1 BJT Circuit Configurations V be ~ ~ ~ v s R L v s R L V Vcc R s cc R s v s R s R L V cc Common base Common emitter Common collector

2 Common emitter current gain

3 BJT Current-Voltage Characteristics V CE, V Very small base current (~5-75 μa) causes much higher collector current (up to 7.5 ma). The current gain is ~ 100

4 BJT amplifier circuit analysis: Operating point R L V CE, V 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 I base = 40 μa, I coll = 4 ma for any E C-E greater than 1.5 V

5 BJT amplifier circuit analysis: Operating point V CC R L V CE V CE, V For an arbitrary collector voltage, collector current can be found using the KVL. The KVL for the collector emitter circuit, V CC = I RL R L + V CE ; I V V CC CE RL = The R L current depends linearly on the collector voltage V CE RL Resistor R L and the C-E circuit of BJT are connected in series, hence I RL = I C For I b = 40 μa and V CC = 13V, the collector current I C = 4 ma For I b = 75 μa and V CC = 14V, the collector current I C = 4 ma

6 BJT amplifier gain analysis: 1 1. Input circuit The input voltage has two components: the DC bias and the AC signal V in 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.

7 BJT amplifier gain analysis: 2 V CE, V 2. Output circuit The collector current has two components too. Base current I C AC current amplitude DC current Time DC and AC collector currents flow through the BJT in accordance with its I-V characteristics

8 BJT amplifier gain analysis: 3 V CE, V The resistance of the B-E junction is very low when V BE V BE0 V bi 0.7 V. Hence the base current I B (V in -V BE0 )/R1 = (V indc -V BE0 +V inac )/R 1 The collector current I C does not depend on the collector voltage if the latter is high enough. Hence, I C βi B ; The voltage drop across the load resistance R 2 : V 2 = I C R 2 ; The output voltage V out = V CC -V 2 = V CC -I C R 2 ; V out = V CC -I C R 2 = V CC - β I B R 2 = V CC - β R 2 (V indc -V BE0 +V inac )/R 1 ; In signal amplifiers only AC component of the output voltage is important: V outac = - β R 2 V inac /R 1 ; The amplifier voltage gain: k V = V outac /V inac = - β R 2 /R 1 ;

9 BJT amplifier gain analysis: 4 V CE, V Common emitter gain summary: Current gain: k I = β Voltage gain: k V = V outac /V inac = - β R 2 /R 1 ; Power gain: k P = k V k I = - β 2 R 2 /R 1

10 BJT design and factors affecting the performance

11 Base resistance and emitter current crowding in BJTs The voltage drop along the base layer V bb = r bb I b, where r bb is called the base spreading resistance. The p-n junction current decreases rapidly when the voltage drops by ΔV bb ~ V th = kt/q Δd e qv V kt VTH I = IS e 1 = IS e 1 The length of the edge region, Δd e, where most of the emitter current flows may be estimated from: V = I R I ρ th b bmax b b Δ d e e tw

12 Emitter current crowding in BJTs (cont.) Δd e V = I R I th b bmax b b ρ = b 1 qn b b μ From these, ρ Δ d e e tw Δ V t W V d = qn μ t W th e th e b b e b Ib ρb Ib

13 Example Δd e Estimate the effective emitter length, Δd e for the BJT having the following parameters: I b = 50 μa; N b = cm -3 μ b = 400 cm 2 /V-s W b = 0.1 μm t e = 100 μm V Δd qn μ t W th e b b e b Ib Δd e = 3.33 e-4 cm = 3.33 μm

14 Large periphery B JT Design

15 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!)

16 In the BJT, this effect means that the effective width of the base is less than W b : W eff = W b -X deb -X dcb where W b is the physical thickness of the base, x deb and x dcb are the depletion region widths from the emitter and collector sides. The depletion widths are given by (N e >>N b ): x x deb dcb 2εε = 0 ( Vbi Vbe ) q N b 2εε + = 0 ( Vbi Vcb ) 2 qnb 1/ 2 N c 1/ 2 x dcb is usually the most important factor since the voltage applied to collector is high. The doping level in collector, N DC has to be lower than that of the base, N AB, to reduce the Early effect: N C << N B

17 Due to Early effect the effective base thickness depends on the collector voltage. In the gain expression, W should be replaced with W eff : β = rec 2 2 L p 2 eff W The Early effect decreases the output resistance, and hence the voltage gain of BJTs.

18 Punch-through breakdown in BJTs - - the result of the Early effect The punch-through breakdown voltage V pt : W= x dcb V pt 2 qw = 2εε 0 N b ( N + N ) c N c b

19 I e BJT Model Gummel-Poon model used in SPICE and other simulators I c Applying KCL to the BJT terminals: I e = I c + I b I b Hence, Common emitter current gain is defined as: Collector emitter current relationship: I c e where α is called a common base current gain I = α I I = α ( I + I ) c c b c I c α = I 1 α = β I b b β= α 1 α α= β 1 +β The last two expressions link common emitter and common base current gains

20 Simplified Gummel-Poon BJT equivalent circuit I e α i I c α n I e Ic Emitter Ideal diode Ideal diode Collector Leakage diode Leakage diode Emitter junction: I be I se V be = exp 1 βf nv F th Base Collector junction: V th = kt/q = V at 300 K I bc I sc V bc = exp 1 βr nv R th

21 Gummel-Poon BJT equivalent circuit accounting for the leakage currents I e α i I c α n I e Ic Emitter Ideal diode Ideal diode Collector Leakage diode Leakage diode I Emitter base leakage diode: V be _ = I exp 1 nv E th Leak be se Base Collector base leakage diode: I V bc _ = I exp 1 nv C th Leak bc sc