Ch. 8 Feedback 1 ECE 352 Electronics II Winter 2003 * Feedback circuit does not load down the basic amplifier A, i.e. doesn’t change its characteristics Doesn’t change gain A Doesn’t change pole frequencies of basic amplifier A Doesn’t change R i and R o * For this configuration, the appropriate gain is the TRANSCONDUCTANCE GAIN A = A Co = I o /V i * For the feedback amplifier as a whole, feedback changes midband transconductance gain from A Co to A Cfo * Feedback changes input resistance from R i to R if * Feedback changes output resistance from R o to R of * Feedback changes low and high frequency 3dB frequencies Series-Series Feedback Amplifier - Ideal Case Co f Co Cfo A A A 1 Co f i if A R R 1 Co f o of A R R 1 Co f L Lf H Co f Hf A A 1 1 Output current sampling oltage fedback to input
Series-Series Feedback Amplifier - Ideal Case. Voltage fedback to input. Feedback circuit does not load down the basic amplifier A, i.e. doesn’t change its characteristics Doesn’t change gain A Doesn’t change pole frequencies of basic amplifier A - PowerPoint PPT Presentation
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Ch. 8 Feedback 1ECE 352 Electronics II Winter 2003
* Feedback circuit does not load down the basic amplifier A, i.e. doesn’t change its characteristics
Doesn’t change gain A Doesn’t change pole frequencies of basic
amplifier A Doesn’t change Ri and Ro
* For this configuration, the appropriate gain is the TRANSCONDUCTANCE GAIN A = ACo = Io/Vi
* For the feedback amplifier as a whole, feedback changes midband transconductance gain from ACo to ACfo
* Feedback changes input resistance from Ri to Rif
* Feedback changes output resistance from Ro to Rof
* Feedback changes low and high frequency 3dB frequencies
Series-Series Feedback Amplifier - Ideal Case
Cof
CoCfo A
AA
1
Cofiif ARR 1
Cofoof ARR 1
Cof
LLfHCofHf A
A
11
Output current sampling
Voltage fedback to input
Ch. 8 Feedback 2ECE 352 Electronics II Winter 2003
Series-Series Feedback Amplifier - Ideal Case
Gain (Transconductance Gain)
Cof
Co
i
of
Co
i
f
Co
fi
iCo
s
oCfo A
A
V
IA
V
VA
VV
VA
V
IA
111
Input Resistance
Cofi
ii
ofi
i
fi
i
sif AR
RV
IV
I
VV
I
VR
1
Output Resistance
Cofot
of
oCoft
otfCotoiCot
tfitfoff
fis
tof
ARI
VRso
RAI
RIAIRVAIV
IVsoIIVand
VVsoVBut
I
VR
1
1
0
V
V+
-
Ch. 8 Feedback 3ECE 352 Electronics II Winter 2003
* Feedback network is a two port network (input and output ports)
* Can represent with Z-parameter network (This is the best for this feedback amplifier configuration)
* Z-parameter equivalent network has FOUR parameters
* Z-parameters relate input and output currents and voltages
* Two parameters chosen as independent variables. For Z-parameter network, these are input and output currents I1 and I2
* Two equations relate other two quantities (input and output voltages V1 and V2) to these independent variables
* Knowing I1 and I2, can calculate V1 and V2 if you know the Z-parameter values
* Z-parameters have units of ohms !
Equivalent Network for Feedback Network
Ch. 8 Feedback 4ECE 352 Electronics II Winter 2003
* Feedback network consists of a set of resistors
* These resistors have loading effects on the basic amplifier, i.e they change its characteristics, such as the gain
* Can use z-parameter equivalent circuit for feedback network
Feedback factor f given by z12 since
Feedforward factor given by z21 (neglected)
z22 gives feedback network loading on output
z11 gives feedback network loading on input
* Can incorporate loading effects in a modified basic amplifier. Gain ACo becomes a new, modified gain ACo’.
* Can then use analysis from ideal case
Series-Series Feedback Amplifier - Practical Case
fo
f
II
V
I
Vz
02
112
1
'1'1
'1''1''1
'
Cof
LLfHCofHf
CofoofCofiifCof
CoCfo
AA
ARRARRA
AA
Ch. 8 Feedback 5ECE 352 Electronics II Winter 2003
Series-Series Feedback Amplifier - Practical Case
* How do we determine the z-parameters for the feedback network?
* For the input loading term z11 We turn off the feedback signal by
setting Io = 0 (I2 = 0 ). We then evaluate the resistance seen
looking into port 1 of the feedback network (R11 =z11).
* For the output loading term z22 We open circuit the connection to the
input so I1 = 0. We find the resistance seen looking
into port 2 of the feedback network (R22 =z22).
* To obtain the feedback factor f (also called z12 )
We apply a test signal Io’ to port 2 of the feedback network and evaluate the feedback voltage Vf (also called V1 here) for I1 = 0.
Find f from f = Vf/Io’
Ch. 8 Feedback 6ECE 352 Electronics II Winter 2003
Series-Series Feedback Amplifier - Practical Case
* Modified basic amplifier (including loading effects of feedback network)
Including z11 at input
Including z22 at output
Including loading effects of source resistance Including load effects of load resistance
* Now have an idealized feedback network, i.e. produces feedback effect, but without loading effects
* Can now use feedback amplifier equations derived
* Note ACo’ is the modified transconductance gain
including the loading effects of z11 , z22 , RS and RL.
Ri’ and Ro’ are modified input and output resistances including loading effects.
'1'1
'1''1''1
'
Cof
LLfHCofHf
CofoofCofiifCof
CoCfo
AA
ARRARRA
AA
Original Amplifier
Feedback Network
Modified Amplifier
Idealized Feedback Network
Ch. 8 Feedback 7ECE 352 Electronics II Winter 2003