Chapter 8 Flashcards
An op amp is a circuit that provides
A high voltage gain and an output proportional to the difference between the two inputs
Op amp producing a moderate output swing requires
Only a small input difference due to its high voltage gain
Inverting amplifier configuration
Inverting input - virtual ground
Integrator applications
Analog filters, analog-to-digital converters
Differentiators
High noise
An inverting configuration using multiple input resistors tied to virtual ground node can serve as
A voltage adder
Placing a diode around an op amp leads to a
Precision rectifier (can rectify very small input swings)
Placing a bipolar device around an op amp provides
A logarithmic function
Op amps suffer from various imperfections
Dc offsets
Input bias currents
The speed of op amps are limited by
The bandwidth of op amps, finite slew rate for large signals
Op-amp applications
Amplifiers, limiters, oscillators
Ideal op-amp properties
Rin = infinity V1=V2 (negative feedback) Ao = infinity Slew rate infinite Rout = 0
Unity gain buffer
If Ao increases, Vout = V1
Non-inverting amplifier
Such a configuration makes it immune to manufacturing defects
Less affected by variations in resistance because gain is function of R1/R2
Virtual ground
Can’t sink current (Vx)
Open loop gain
Ao
Closed loop gain
Av
Integrator
Pole at origin
Acts as a low pass filter
Slope past pole is constant -20dB/dec
The pole can be found by
Setting denominator to 0
Differentiator (Ao = infinity)
Zero at origin
Slope of H(s) changed by +20dB/dec after a zero
High pass filter
Non-linear function
Precision rectifier
Logarithmic amplifier
Square root amplifier
Precision rectifier
Similar to unity gain buffer (w/o diode)
Normal diode based rectifiers need the input to be at least 0.7V to forward bias the diode
Vin=0, Vout=0.7V and Vout=Vin
-as Vin increases, Vout rises as well and Vout=Vin
Vout - drop across R1
Negative cycle of input - current should flow from ground to R to diode to op-amp
(Not possible due to the diode, Vout=0)
Logarithmic amplifier
Vin negative - circuit will not operate (open loop circuit)
Square root amplifier
If Vin increases, then Vout is more negative, Vgs more positive, and Vgs > Vt. MOSFET operates in saturation and I increases
Input bias current
Op-amps are generally based on BJTs, which tend to draw small base current (which may cause inaccuracies)
Offset voltage
Couple capacitors block DC, hence offset voltage
Slew rate
V/us
Slope of maximum allowable ramp
Occurs when rate is too high and the fastest the op-amp can respond (causing delay and distortion)
Input high - rate will be high as well so that may lead to output lagging input
Generally 10V/us
Happens because internal circuit acts like current source charging capacitors
Op-amp bandwidth
Open loop gain not independent of frequency
Finite BW makes it act like a capacitive circuit, gain decreases
Non-infinite Ao pushes the pole further (BW increases, gain decreases)
Higher the gain, the unity gain BW will be low
Unity gain BW
Output = input
Not useful is below 1 (not amplifying)
