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Electronic Amplifiers
Amplifier: activ three-port network that delivers to the output a
signal xo(t) (voltage or current) with the same shape as the input
signal xi(t) and can provide greater power on an adequate load.
 Linear circuit:
x0 proportional with xi
A – amplification, gain
xo (t )  Axi (t )
A<0 inverting
A>0
non-inverting
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The Amplifier Supply
 dc voltage and/or current sources
 more frequently – dc voltage sources
Single source supply
 Two source supply
(symmetric differential)
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Power transfer and power balance
 the average power of the output signal Pout is greater than the
average power of the input signal Pin.
 the excess of the output power is taken from the supply sources
Psupply+Pin=Pout+Pdissip
Psupply  Pout+Pdissip
 efficiency
η =Pout/Psupply
 a step up transformer
is not an amplifier
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Amplifier types
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VTC for a voltage-to-voltage noninverting
amplifier, symmetric differential supply
 amplification (active) region:
 VOL VOH 
;
vI  
;
 Av Av 
vO  VOL ;VOH 
ideal amplifier:
VOL=-VPS
VOH=+VPS
 general-purpose OA
vO  (VPS  1V...2V;
 VPS  1V...2V)
 rail-to-rail OA:
vO   VPS ;VPS 
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Signal
transfer
Remark: input signal - low
enough for the amplifier to work
in a linear region around the OP:
small signal approximation
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Amplifier models
 two-port network: it consider explicitly only the behavior to the
input and output ports and input-output transfer for signal the signal
 valid irrespective of the internal complexity of the amplifiers
 valid in the bandpass frequency domain
Linear controlled sources
 active two-port network – only one finite, non-zero parameter :
dforward transfer parameter (gain)
 the output signal is controlled by the input signal
 pseudo-sources
Example: VCVS
vO = avvi
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Modeling the voltage amplifier
vo
av 
vi
Ri – draws current from
the input signal source
Ro – deteriorates the
output voltage in the
presence of load (voltage
divider)
vo
Av 
vs
Ri
RL
Av 
av
Rs  Ri RL  Ro
Ideal amplifier ?
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Av is closer to the open circuit gain av, when one can reduce the
voltage losses to the input (across Rs) and to the output (across Ro)
 Ri>>Rs – the source voltage
appears to the input of the amplifier
vi  vs
 Ro<<RL - the voltage of the
controlled voltage appear to the output
vo  avvi
ideal voltage amplifier
Ri = ∞;
Ro = 0
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Determining the amplifier performances
 gain (forward transfer factor)
 input resistance
 output resistance
Gain
• analysis the circuit using theorems and electrical circuit relations
(Kirchhoff, Ohm, etc.) and equations describing the operation of the
active devices
 express the output signal as a function of the input signal and
compute the gain
Input resistance
vi
Ri 
ii
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Output resistance
 Set the input signal
source to zero
1.
 Connect to the output a
test source
vtest
Ro 
itest
2.
open
short-circuit
Ro 
vo ,open
io ,sc
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Frequency response
 analyze the equivalent model of the amplifier including capacitive
components too, by their complex impedances 1/jωC
 Complex transfer function for the gain:
vo ( j )
A( j ) 
vi ( j )
 input and output impedances are replaced by their complex
counterparts Z i ( j ) Zo  j 
Band-pass
amplifier
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 the decrease of the gain to high frequency - internal capacitances
(parasitic) of the active devices
 the decrease of the gain to low frequency – coupling / decoupling
capacitors (usual fractions – tens of F)
 dc coupling (direct)  LPF
capacitive
coupling at input
capacitive coupling of
two amplifier stages
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