eeet2555 rmit electronics

8/10/2019 EEET2555 RMIT ELECTRONICS

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Small Signal Model

Voltage Amplifier (Voltage in – Voltage out)How to find R i?

ii

i

v R

i=

How to find R O? (Source = short circuit)

0i

xo

x v

v R

i =

=

How to find A Vo? (Output = open circuit)

ovo

i open

v A

v=

Current Amplifier (Current in – Current out)How to find R i?

ii

i

v Ri

=


0i

xo

x i

v R

i =

=

How to find A is? (Output = short circuit)

ois

i short

i A

i=

Relationship with A Vo

ovo is

i

R A A

R

=

Transconductance Amplifier (Voltage in – Current out) How to find R i?

ii

i

v R

i=


0i

xo

x i

v R

i =

=

How to find A is? (Output = short circuit)

ois

i short

i A

i=


vo m o A G R=



Transresistance Amplifier (Current in – Voltage out) How to find R i?

ii

i

v R

i=


0i

xo

x v

v R

i =

=

How to find A Vo? (Output = open circuit)

ovo

i open

v A

v=


mvo

i

R A

R=

Suggested solution steps (Gain and R in):

1. Sketch the small signal model of the circuit.2. Define A, and express it as 3 product of two terms, one fully defined by the input side and the other

by the output side.3. Consider input and output circuits separately and derive (in pronumerals) the expressions for both

terms of Step 2.4. Express Av as the product of both parts from Step 3.5. Substitute the numerical values into the AL, expression from Step 4 and calculate the gain value.6. Consider the input side of the small signal model from Step 1 and establish, which is defined as R in =

Vs/Is.

Q1. BJT Amplifier Biasing Condition (Related to Lab)

1. Find IB by using KVL (Assume V BE = 0.7V)2. Find IC and I E by using3. Find VE by using V=IR4. Find Vc and VCE by using KVL



For a 4 resistors Common Emitter Biasing, Find the Thevenin Equivalent Circuit for the base

Finding Q-Point (Q-point is the point (Current and Voltage) where the amplifier operates.)

1. Sketch the output characteristic2. Find the load line (at the collector circuit)3. Determine the Q-Point



Q2. BJT Amplifier Small Signal Operation (Related to Lab)

1. Replace the BJT with SSM2. Short circuit the DC source3. Short circuit the capacitor



Note:

be b b

ebe b

bbe e

be e e

b e

v r i

iv r

r v i

Sincev r i

hence

r r

β

β

β

=

=

=

=

=

T e

EQ

T

B

V r

I

or

V r

I π

=

=



Q3. Amplifier with Feedback (Test 2)

If short circuit the RL kill the feedback

– Voltage sampling

If open circuit the RL kill the feedback – Current sampling



Walk through the source1. No feedback – Current Mixing2. Has feedback – Voltage Mixing



Q4. BJT SSM Frequency Response (Lab 4)

Steps:

1. DC analysis (Find IB, IE and IC (and VB, VE and VC if requested))

2. Establish the SSM (Find r π, ro and g m (and r e if requested))3. AC analysis (Find Rin, Rout and AV)

4. Find the low break frequency for the external capacitors (f = f 1 + f 2 + f 3)

5. Find the high break frequency for the internal resistor (1 2 3

1 1 1 1 f f f f

= + + )

a. Determine the Miller voltage gain.

b. Determine inC µ and out C µ .

c. Find the high break frequency from the SSM of the Hybrid π Model (with Miller Effect)



Additional Notes

For CE

1|| ( ) || ( )1 1Th E in out R R R R

β β β

=+ +

For CS

( )1 2|| || ( ) || (( 1)Th s b b i out EM R R R R R R R β β = + + +

For CC

1 1( || || ( ) || ( ) )Th L C o in EM R R R R R R β

β β

+= +

eeet2555 rmit electronics

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