Lecture 22

Cascaded Systems and Combination BJT-FET Circuits

BJT-FET 1-1

Outline

Introduction to Cascaded Amplifier Systems

Introduction to BJT-FET Combination Circuits

BJT-FET 1-2

Cascaded Amplifier Systems

A cascade amplifier is any two-port network constructed from a series of amplifiers, where each amplifier sends its output to the input of the next amplifier stage

The overall voltage gain is determined by the product of gains of the individual stages

The DC bias circuits are isolated from each other by the coupling capacitors

The DC calculations are independent of the cascading

The AC calculations for gain and impedance are interdependent

BJT-FET 1-3

R-C Coupled BJT Amplifiers

BJT-FET 1-4

2Co RZ

Input impedance, first stage:

Output impedance, second stage:

Voltage gain:

ei rRRZ |||| 21

21

22

4311

||||||

vvv

e

CV

e

eCv

AAA

r

RA

r

rRRRA

Multistage Amplifiers

BJT-FET 1-5

Problem

BJT-FET 1-6

The figure shows two-stage RC coupled amplifier. If the input resistance Rin of each stage is 1 kΩ, find: (i) voltage gain of first stage (ii) voltage gain of second stage (iii) total voltage gain.

Cascode Amplifier Systems

Cascode amplifier

High frequency amplifier made up of a common-emitter amplifier with a common-base amplifier in its collector network

BJT-FET 1-7

Comparisons between MOSFETs and BJTs

BJT-FET 1-8

MOSFETs BJTs

Pros Cons

High input impedance Low input impedance

Minimal drive power, no DC current required at gate

Large drive power, continuous DC current required at base

Simple drive circuits Complex drive circuits as large +ve and –ve currents are involved

Devices can be easily paralleled Devices cannot be easily paralleled

Max. operating temp. ~ 200 oC , less temp. sensitive

Max. operating temp. ~ 150 oC , more sensitive to temp

Very low switching losses Medium to high switching losses (depends on trade-off with conduction losses)

High switching speed Lower switching speed

Cons Pros

High on-resistance Low on-resistance

Low transconductance High transconductance

BJT-FET Combination Circuits

Combination of BJT and FET device in a circuit Innovative circuits that take some advantages of FETs,

such as the high-input-impedance and low input power operation, and some merits of BJTs, such as high output current-driving capability

How to analyze such circuits Firstly, recognize both of the devices and their current

flows

To make the calculation simple and easier to view, transform the circuit into the equivalent form to avoid complexity

List down all the important relationships that involve for both of the devices

Start with approaching the device that is closer to the ground (bottom device)

BJT-FET 1-9

Example (3)

BJT-FET 1-10

Determine VD and VC

Example (3) – Solution

BJT-FET 1-11

We know that for the JFET device, IG = 0 making the resistor RG = 1 MΩ useless and can be remove from the circuit

By analyzing the circuit, we notice that the configuration is a voltage-divider bias for both the JFET and BJT device

Due to involvement of BJT, we have to check βRE ≥ 10R2 to use the approximate analysis

As for βRE = (180)(1.6k) = 288k and 10R2 = 10(24k) = 240k, situation βRE ≥ 10R2 is satisfied and we can use approximate analysis for this configuration

Obtaining the ETH:

V 62.38224

24*16ETH

Example (3) – Solution

BJT-FET 1-12

ETH = 3.62 V

Transforming the circuit into its equivalent form:

ETH = 3.62 V

Example (3) – Solution

BJT-FET 1-13

By approaching BJT (bottom device) first, we know VBE = 0.7 active operating mode

From earlier calculation, we got ETH = VB = 3.62 V

Obtaining VE:

Obtaining IB from VBE = 0.7:

1

181 1.6 289.6

E E E B E

B B

V I R I R

I I

A 08.10I

kI6.28962.37.0

7.0VVV

B

B

EBBE

Example (3) – Solution

BJT-FET 1-14

From the circuit, IB is not really important but IC is very important because

IC = IS = ID

As for that, obtain IC:

Knowing the value of ID, VD can be obtained:

mA 81.1)08.10)(180( BC II

V 11.11107.216 3 DD IV

ETH = 3.62 V

Example (3) – Solution

BJT-FET 1-15

From the configuration, we notice that VS = VC

By obtaining VGS for the JFET, the value of VS can be achieved:

V 29.7

6

62.311281.1

1

62.362.3

2

2

C

C

P

GSDSSD

CSSGGS

V

Vmm

V

VII

VVVVV

ETH = 3.62 V

The Darlington Pair

BJT-FET 1-16

- The Darlington circuit provides a very high current gain—the product of the individual current gains:

D = 12

- The practical significance is that the circuit provides a very high input impedance.

DC Bias of Darlington Circuits

BJT-FET 1-17

BDBDE II)1(I

EEE RIV

EDB

BECCB

RR

VVI

Base current:

Emitter current:

Emitter voltage:

Base voltage:

BEEB VVV

D = 12

A Darlington emitter-follower used as a buffer between a common-emitter amplifier and a low-resistance load such as a speaker

BJT-FET 1-18

The Sziklai Pair “Complementary Darlington”

BJT-FET 1-19

- one NPN and one PNP transistor - Current gain is similar to that of a Darlington pair - The base turn-on voltage is only about half of the Darlington's turn-on voltage

Current Mirror Circuits

BJT-FET 1-20

- Current mirror circuits provide integrated circuits with constant current, regardless of loading

- The current mirror is used to provide bias currents and active loads to circuits

- Applications: - Voltage to current

converters - Current-mode analog signal

processing (low-voltage operation)

A Simple Current Source (Current Mirror)

BJT-FET 1-21

Take Home Problem Prove that the following common-collector BJT circuit has a negative feedback effect

BJT 1-22

BJT-FET 1-23

Lecture Summary

Covered material Introduction to cascaded systems using BJT Introduction to BJT-FET Combination Circuits

Combination of BJT and FET device in a circuit