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Recall Lecture 13 Biasing of BJT Three types of biasing Fixed Bias Biasing Circuit Biasing using Collector to Base Feedback Resistor Voltage Divider Biasing Circuit Applications of BJT As digital logic gates NOT NOR

Recall Lecture 13 Biasing of BJT Three types of biasing Fixed Bias Biasing Circuit Biasing using Collector to Base Feedback Resistor Voltage Divider Biasing

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Page 1: Recall Lecture 13 Biasing of BJT Three types of biasing Fixed Bias Biasing Circuit Biasing using Collector to Base Feedback Resistor Voltage Divider Biasing

Recall Lecture 13

Biasing of BJT Three types of biasing

Fixed Bias Biasing Circuit Biasing using Collector to Base Feedback Resistor Voltage Divider Biasing Circuit

Applications of BJT As digital logic gates

NOT NOR

Page 2: Recall Lecture 13 Biasing of BJT Three types of biasing Fixed Bias Biasing Circuit Biasing using Collector to Base Feedback Resistor Voltage Divider Biasing

CHAPTER 5BASIC BJT AMPLIFIERS

(AC ANALYSIS)

Page 3: Recall Lecture 13 Biasing of BJT Three types of biasing Fixed Bias Biasing Circuit Biasing using Collector to Base Feedback Resistor Voltage Divider Biasing

The Bipolar Linear AmplifierThe Bipolar Linear Amplifier

Bipolar transistors have been traditionally used in linear amplifier circuits because of their relatively high gain.

To use the circuit as an amplifier, the transistor needs to be biased with a dc voltage at a quiescent point (Q-point) such that the transistor is biased in the forward-active region.

If a time-varying signal is superimposed on the dc input voltage, the output voltage will change along the transfer curve producing a time-varying output voltage.

If the time-varying output voltage is directly proportional to and larger than the time-varying input voltage, then the circuit is a linear amplifier.

Page 4: Recall Lecture 13 Biasing of BJT Three types of biasing Fixed Bias Biasing Circuit Biasing using Collector to Base Feedback Resistor Voltage Divider Biasing

The linear amplifier applies superposition principle Response – sum of responses of the circuit for

each input signals alone So, for linear amplifier,

DC analysis is performed with AC source turns off or set to zero

AC analysis is performed with DC source set to zero

Page 5: Recall Lecture 13 Biasing of BJT Three types of biasing Fixed Bias Biasing Circuit Biasing using Collector to Base Feedback Resistor Voltage Divider Biasing

EXAMPLE

iC , iB and iE,

vCE and vBE

Sum of both ac and dc components

Page 6: Recall Lecture 13 Biasing of BJT Three types of biasing Fixed Bias Biasing Circuit Biasing using Collector to Base Feedback Resistor Voltage Divider Biasing

Graphical Analysis and ac Equivalent CircuitGraphical Analysis and ac Equivalent Circuit

From the concept of small signal, all the time-varying signals are superimposed on dc values. Then:

and

Page 7: Recall Lecture 13 Biasing of BJT Three types of biasing Fixed Bias Biasing Circuit Biasing using Collector to Base Feedback Resistor Voltage Divider Biasing

PERFORMING DC and AC analysis

DC ANALYSIS AC ANALYSIS

Turn off AC SUPPLY = short circuit

Turn off DC SUPPLY = short circuit

Page 8: Recall Lecture 13 Biasing of BJT Three types of biasing Fixed Bias Biasing Circuit Biasing using Collector to Base Feedback Resistor Voltage Divider Biasing

DO YOU STILL REMEMBER?

Page 9: Recall Lecture 13 Biasing of BJT Three types of biasing Fixed Bias Biasing Circuit Biasing using Collector to Base Feedback Resistor Voltage Divider Biasing

DC equivalent AC equivalent

rd

idIDQ

VDQ = V

Let’s assume that Model 2 is used

Page 10: Recall Lecture 13 Biasing of BJT Three types of biasing Fixed Bias Biasing Circuit Biasing using Collector to Base Feedback Resistor Voltage Divider Biasing

AC equivalent circuit –

Small-Signal Hybrid-Small-Signal Hybrid-ππ Equivalent Equivalent

ib

OR

Page 11: Recall Lecture 13 Biasing of BJT Three types of biasing Fixed Bias Biasing Circuit Biasing using Collector to Base Feedback Resistor Voltage Divider Biasing

THE SMALL SIGNAL PARAMETERS

The resistance rπ is called diffusion resistance or B-E input resistance. It is connected between Base and Emitter terminals

The term gm is called a transconductance

ro = VA / ICQrO = small signal transistor output resistance

VA is normally equals to , hence, if that is the case, rO = open circuit

Page 12: Recall Lecture 13 Biasing of BJT Three types of biasing Fixed Bias Biasing Circuit Biasing using Collector to Base Feedback Resistor Voltage Divider Biasing

Hence from the equation of the AC parameters, we HAVE to perform DC analysis first in order to calculate them.

Page 13: Recall Lecture 13 Biasing of BJT Three types of biasing Fixed Bias Biasing Circuit Biasing using Collector to Base Feedback Resistor Voltage Divider Biasing

EXAMPLE

The transistor parameter are = 125 and VA=200V. A value of gm = 200 mA/V is desired. Determine the collector current, ICQ and then find r and ro

ANSWERS: ICQ = 5.2 mA, r= 0.625 k and ro = 38.5 k

Page 14: Recall Lecture 13 Biasing of BJT Three types of biasing Fixed Bias Biasing Circuit Biasing using Collector to Base Feedback Resistor Voltage Divider Biasing

CALCULATION OF GAIN

Voltage Gain, AV = vo / vs

Current Gain, Ai = iout / is

Page 15: Recall Lecture 13 Biasing of BJT Three types of biasing Fixed Bias Biasing Circuit Biasing using Collector to Base Feedback Resistor Voltage Divider Biasing

Small-Signal Voltage Gain: Av = Vo / Vs

ib

Page 16: Recall Lecture 13 Biasing of BJT Three types of biasing Fixed Bias Biasing Circuit Biasing using Collector to Base Feedback Resistor Voltage Divider Biasing

Common-Emitter Common-Emitter AmplifierAmplifier

Page 17: Recall Lecture 13 Biasing of BJT Three types of biasing Fixed Bias Biasing Circuit Biasing using Collector to Base Feedback Resistor Voltage Divider Biasing

Remember that for Common Emitter Amplifier, the output is measured at the collector terminal. the gain is a negative value

Three types of common emitter Emitter grounded With RE

With bypass capacitor CE

Page 18: Recall Lecture 13 Biasing of BJT Three types of biasing Fixed Bias Biasing Circuit Biasing using Collector to Base Feedback Resistor Voltage Divider Biasing

STEPSOUTPUT SIDE

1.Get the equivalent resistance at the output side, ROUT

2.Get the vo equation where vo = - ib ROUT

INPUT SIDE

3.Calculate Rib using KVL where Rib = vb / ib

4.Calculate Ri

5.Get vb in terms of vs – eg: using voltage divider.

6.Go back to vo equation and replace where necessary

Page 19: Recall Lecture 13 Biasing of BJT Three types of biasing Fixed Bias Biasing Circuit Biasing using Collector to Base Feedback Resistor Voltage Divider Biasing

VCC = 12 V

RC = 6 k93.7 k

6.3 k

0.5 kβ = 100VBE = 0.7VVA = 100 V

Emitter Grounded

Voltage Divider biasing:Change to Thevenin EquivalentRTH = 5.9 kVTH = 0.756 V

Page 20: Recall Lecture 13 Biasing of BJT Three types of biasing Fixed Bias Biasing Circuit Biasing using Collector to Base Feedback Resistor Voltage Divider Biasing

Perform DC analysis to obtain the value of IC

BE loop: 5.9IB + 0.7 – 0.756 = 0

IB = 0.00949

IC = βIB = 0.949 mA

Calculate the small-signal parameters

r = 2.74 k , ro = 105.37 k and gm = 36.5 mA/V

Page 21: Recall Lecture 13 Biasing of BJT Three types of biasing Fixed Bias Biasing Circuit Biasing using Collector to Base Feedback Resistor Voltage Divider Biasing

VCC = 12 V

RC = 6 k93.7 k

6.3 k

0.5 kβ = 100VBE = 0.7VVA = 100 V

Emitter Grounded

Page 22: Recall Lecture 13 Biasing of BJT Three types of biasing Fixed Bias Biasing Circuit Biasing using Collector to Base Feedback Resistor Voltage Divider Biasing

vb

Follow the steps1. Rout = ro || RC = 5.677 k

2. Equation of vo : vo = - ( ro || RC ) ib= - 567.7 ib

4. Calculate Ri RTH||r = 1.87 k

5. vb in terms of vs use voltage divider: vb = [ Ri / ( Ri + Rs )] * vs = 0.789 vs

3. Calculate Rib using KVL: ib r - vb = 0 hence Rib = r

Page 23: Recall Lecture 13 Biasing of BJT Three types of biasing Fixed Bias Biasing Circuit Biasing using Collector to Base Feedback Resistor Voltage Divider Biasing

so: vb = 0.789 vs replace in equation from step 1

6. Go back to equation of vo

vo = - 567.7 ib

but ib = vb / Rib (from step 3)

vo = - 567.7 [0.789 vs] / 2.74

vo = -163.5 vs

AV = vo / vs = - 163.5

vb

bring VS over

Page 24: Recall Lecture 13 Biasing of BJT Three types of biasing Fixed Bias Biasing Circuit Biasing using Collector to Base Feedback Resistor Voltage Divider Biasing

TYPE 2: Emitter terminal connected with RE – normally ro = in this type

VCC = 5 V

RC = 5.6 k250 k

75 k

0.5 k

RE = 0.6 k

β = 120VBE = 0.7VVA =

user
Page 25: Recall Lecture 13 Biasing of BJT Three types of biasing Fixed Bias Biasing Circuit Biasing using Collector to Base Feedback Resistor Voltage Divider Biasing

0.5 k

57.7 k

RC = 6 k

7.46 k

RE = 0.6 k

vb

Page 26: Recall Lecture 13 Biasing of BJT Three types of biasing Fixed Bias Biasing Circuit Biasing using Collector to Base Feedback Resistor Voltage Divider Biasing

1. Rout = RC = 6 k

2. Equation of vo : vo = - RC ib= - 720 ib

4. Calculate Ri RTH||Rib = 33.53 k

5. vb in terms of vs use voltage divider: vb = [ Ri / ( Ri + Rs )] * vs = 0.9853 vs

3. Calculate Rib using KVL: ib r + ie RE - vb = 0

but ie = (1+ ) ib = 121 ibso: ib [ 121(0.6) + 7.46 ] = vb Rib = 80.06 k

vb

Page 27: Recall Lecture 13 Biasing of BJT Three types of biasing Fixed Bias Biasing Circuit Biasing using Collector to Base Feedback Resistor Voltage Divider Biasing

so: vb = 0.9853 vs

6. Go back to equation of vo

vo = - 720 ib = - 720 [ vb / Rib ]

vo = - 720 [ 0.9853 vs / 80.06 ]

vo = - 8.86vs

AV = vo / vs = - 8.86bring VS over

vb

Page 28: Recall Lecture 13 Biasing of BJT Three types of biasing Fixed Bias Biasing Circuit Biasing using Collector to Base Feedback Resistor Voltage Divider Biasing

Circuit with Emitter Bypass Capacitor

● There may be times when the emitter resistor must be large for the purpose of DC design, but degrades the small-signal gain too severely.

● An emitter bypass capacitor can be used to effectively create a short circuit path during ac analysis hence avoiding the effect RE

TYPE 3: With Emitter Bypass Capacitor, CE

Page 29: Recall Lecture 13 Biasing of BJT Three types of biasing Fixed Bias Biasing Circuit Biasing using Collector to Base Feedback Resistor Voltage Divider Biasing

vb

CE becomes a short circuit path – bypass RE; hence similar to Type 1

Page 30: Recall Lecture 13 Biasing of BJT Three types of biasing Fixed Bias Biasing Circuit Biasing using Collector to Base Feedback Resistor Voltage Divider Biasing

VCC = 5 V

RC = 2.3 k20 k

20 k

0 k

RE = 5k

Bypass capacitor

β = 125VBE = 0.7VVA = 200 VIC = 0.84 mA

Page 31: Recall Lecture 13 Biasing of BJT Three types of biasing Fixed Bias Biasing Circuit Biasing using Collector to Base Feedback Resistor Voltage Divider Biasing

0 k

10 kRC = 2.3 k

238 k

3.87 k

vb

β = 125VBE = 0.7VVA = 200 VIC = 0.84 mA

Short-circuited (bypass) by the capacitor CE

Page 32: Recall Lecture 13 Biasing of BJT Three types of biasing Fixed Bias Biasing Circuit Biasing using Collector to Base Feedback Resistor Voltage Divider Biasing

vb

Follow the steps1. Rout = ro || RC = 2.278 k

2. Equation of vo : vo = - ( ro || RC ) ib= - 284.75 ib

4. Calculate Ri RTH||r = 2.79 k

5. vb in terms of vs use voltage divider: vb = [ Ri / ( Ri + Rs )] * vs = vs since RS = 0 k

3. Calculate Rib using KVL: ib r - vb = 0 hence Rib = r = 3.87 k

Page 33: Recall Lecture 13 Biasing of BJT Three types of biasing Fixed Bias Biasing Circuit Biasing using Collector to Base Feedback Resistor Voltage Divider Biasing

so: v b = vs

6. Go back to equation of vo

vo = - 284.75 ib = - 284.75 [ vb / Rib ]

vo = - 284.75 (vs) / 3.87

vo = -73.58 vs

AV = vo / vs = - 73.58bring VS over

vb