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The Operational Amplifier continued
Note: By summing point
constraint, we have Vi = 0 and ii
= 0.
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The Operational Amplifier continued
The voltage follower provides
unity gain, however, the output
impedance is changed according
to the o/p impedance of the opamp, which is very useful
sometimes. Thus the op-amp acts
as the buffer stage preventing the
o/p load fluctuations to affect the
i/p voltage signal.
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Non-inverting amplifier design: Effects of resistance choice
Av = 10 = 1+R2/R1
For v0 = 10 V, output
current is 1A
Most op amps cannot
handle such large
current,
so small R’s should
be avoided
Very large resistances
tend to be unstable, and
lead to coupling of
unwanted signals
especially at higher
frequencies. Why?
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Op-Amp Imperfections in the linear range of operations
Input and Output Impedances
 An Ideal op amp has infinite input impedance and zero output impedance
 A Real op amp has finite input impedance and nonzero output impedance
 For IC op amps made of BJTs open-loop input impedance is about 1 MW
 For IC op amps made of JFETs open-loop input impedance is about 1012 W
 Open loop output impedance is between 1 and 100 W
 Closed loop impedances will be different, and can be chosen by proper
resistors
Characteristics of two popular Op Amps
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Op-Amp Imperfections in The Linear Range of Operations
Gain and Bandwidth Limitations
 Ideal op amps have infinite open-loop gain magnitude (AoL is infinite), but the gain
of a real op amp is finite and a function of frequency
 dc open-circuit differential voltage gain is typically between 104 to 106
 The bandwidth is usually limited by the designer to prevent oscillations from
feedback, by a process is called frequency compensation
The open loop gain
function of an op-amp
usually has a single
dominant pole and is given
as:
 AOL (f)
 A0OL
 fBOL
 AOL (f)
- open-loop gain as function of frequency
- dc open-loop gain
- open-loop break frequency
- constant up to fBOL then it rolls off at 20
dB/decade
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