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Static
(DC)
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Dynamic
(AC)
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Linear
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Error
Source
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Offset
Voltage
DC Bias Current
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Finite
Bandwidth
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Text
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Sec. 2.7
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Sec. 2.5
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Analysis
Strategy |
- Add DC sources to ideal op-amp to model errors
- Analyze with superposition to find contribution of each
error source to output voltage
- Add in "worst case" fashion
- Offset voltage can be either polarity
- Bias current polarity known from type of op-amp input stage
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Model closed
loop behavior with first-order transfer function:
- DC gain from ideal op-amp assumptions
- Closed loop bandwidth (3-dB frequency)
f3-dB from
gain-bandwdith product relationship
Gain-bandwidth product procedure:
- Redraw circuit with all inputs suppressed (set = 0)
- Find feedback factor β (fraction of output fed back to
inverting input)
- Closed loop bandwidth f3-dB will be unity gain
frequency ft mutliplied by β
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Non-Linear
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Error
Source
|
Output
voltage swing limit
Output current limit
|
Slew rate
limit
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Text
|
Sec. 2.6.1, 2.6.2
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Sec. 2.6.3
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Analysis
Strategy
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- Determine maximum total vOUT, iOUT,
excursion at op-amp output from linear system model
- Compare maximum, minimum to op-amp limits
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- Determine vOUT(t) from linear system behavior
(transfer function for sine wave, general exponential response for step)
- Calculate time derivative dvOUT(t)/dt
- Compare maximum positive and negative dvOUT(t)/dt
to op-amp
slew rate limit
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