Chapter 12
Common Mode
A condition characterized by the presence of the same signal on both op-amp inputs
Gain-bandwidth product
A constant parameter for compensated op-amps which is always equal to the frequnecy at which the op-amps open loop gain is unity
Differential mode
A mode of op-amp operation in which two opposite-polarity signal voltages are applied to the two inputs (double-ended) or in which a signal to one input and ground to other input (single-ended)
Operational amplifier (op-amp)
A type of amplifier that has a very high voltage gain, very high input impedance, very low output impedance, and good rejection of common-mode isgnals
Differential Amplifier
A type of amplifier with two inputs and two outputs that is used as the input stage of an op-amp
A certain noninverting amplifier has an Ri of 1k ohm and Rf of 100k ohm. The closed-loop gain is
A(cl) = 1 + (Rf/Ri) A(cl) = 1+ (100k/1k) A(cl)= 101
Non-inverting amplifier
An op-amp closed-loop configuration in which the input signal is applied to the noninverting input
Inverting amplifier
An op-amp closed-loop configuration in which the input signal is applued to the inverting input
If Rf is decreased in the circuit of Figure 12-19, the voltage gain will
Decrease
If Rf is increased in the circuit of Figure 12-19, the voltage gain will
Decrease
If Ri in Figure 12-44(b) opens, the output voltage will
Decrease
If the closed-loop gain in Figure 12-44(a) is increased by increasing the value of Rf, the closed-loop bandwidth will
Decrease
Negative feedback
Decreases the output impedance and the bandwidth
T/F: CMRR stands for common-mode rejection reference
False, Common-mode rejection ratio
T/F: An ideal op-amp has a very high output impedance
False; An ideal op-amp has infinite voltage gain, infinite bandwidth, infinite input impedance, and zero output impedance.
T/F: Negative feedback reduces the bandwidth of an op-amp from its open-loop value
False; With negative feedback, the op-amp has significantly wider bandwidth
T/F: The gain of a voltage-follower is very high
False; there is no gain
A certain op-amp has bias current of 50 microamps and 49.3 microamps. The input offset current is
I(OS) = | I1-I2 | I(OS)= |50-49.3| I(OS)= 700nA
When negative feedback is used, the gain-bandwidth product of an op-amp
Icreases
In Figure 12-29, if Rf is changed from 100k ohms to 68k ohms, the feedback attenuation will
Increase
If the feedback resistor in (A certain noninverting amplifier has an Ri of 1k ohm and Rf of 100k ohm) is open, the voltage gain
Increases
Which of the following characteristics does not necessarily apply to an op-amp
Low power; op-amp does have high gain, high input impedance, and low output impedance
If Rf is changed to 470k ohms and Ri is changed to 10k ohms in Figure 12-44(b), the closed-loop bandwidth will
Not change
A certain inverting amplifier has a closed-loop gain of 25. The op-amp has an open-loop gain of 100,000. If another op-amp with an open-loop gain of 200,000 is substituted in the configuration, the closed-loop gain
Remains at 25
T/F: Common-mode rejection means that a signal appearing on both inputs is effectively cancelled
True
T/F: If the feedback resistor in an inverting amplifier opens, the gain becomes zero
True
T/F: Negative feedback affects the input and output impedances of an op-amp
True
T/F: Negative feedback reduces the gain of an op-amp from its open-loop value
True
T/F: Slew rate determine how fast the output can change in response to a step input
True
T/F: The gain-bandwidth product equals the unity-gain frequency from a compensated op-amp
True
T/F: The op-amp can operate in either the differential mode or common mode
True
Bias current compensation
reduces output error voltage
The output of a particular op-amp increases 8V in 12 microseconds. The slew rate is
slew rate= change in V/change in time slew rate= 8V/12 slew rate= 0.67 V/miroseconds
Phase shift through an op-amp is caused by
the internal RC circuits
The frequency at which the open-loop gain is equal to 1 is called
the unity-gain frequency
An integrated circuit (IC) op-amp has
two inputs and one output
Common-mode gain is
very low
With zero volts on both inputs, an op-amp ideally should have an output equal to
zero
The purpose of offset nulling is to
zero the output error voltage
Of the values listed, the most realistic value for open-loop gain of an op-amp is
100,000
The bandwidth of a dc amplifier having an upper critical frequency of 100kHz is
100kHz
If a certain compensated op-amp has a mid-range open-loop gain of 200,000 and a unity-gain frequency of 5MHz, the gain-bandwidth product is
5,000,000 Hz
The bandwidth of an ac amplifier having a lower critical frequency of 1kHz and an upper critical frequency of 10kHz is
B= 10l-1k B= 9kHz
If A(ol)=3500 and A(cm)= 0.35, the CMRR is
CMRR= A(ol) / A(cm) CMRR= 3500/0.35 CMRR= 10,000
CMRR
Common Mode Rejection Ratio; the ratio of open-loop gain to common-mode gain; a measure of an op-amps ability to reject common-mode signals
If 10mV is applied to the input to the op-amp circuit of Figure 12-23 and Rf is increased, the output voltage will
Increase
If Vin = 1mV and Rf opens in the circuit of Figure 12-19, the output voltage will
Increase
Negative feedback
The process of returning a portion of the output signal to the input of an amplifier that is out of phase with the input signal
Slew rate
The rate of change of the output voltage of an op-amp in response to a step input
Phase shift
The relative angular displacement of a time-varying fucntion relative to a reference
Open-loop voltage gain
The voltage gain an op-amp without external feedback
Closed-loop voltage gain
The voltage gain of an op-amp with external feedback
T/F: A compensated op-amp has a gain roll-off of -20dB decade above the critical frequnecy
True
T/F: A noninverting amplifier uses negative feedback
True
T/F: An ideal op-amp has an infinite input impedance
True
In the common mode,
an identical signal appears on both inputs
Each RC circuit in an op-amp
causes the gain to roll off at -6dB/octave and causes the gain to foll off at -20dB/decade
The mid-range open-loop gain of an op-amp
extends from 0Hz to the upper critical frequency
If a certain op-amp has a closed-loop gain of 20 and an upper critical frequency of 10MHz, the gain-bandwidth product is
f= A* f f= 20*(10MHz) f= 200MHz and the unity-gain frequency
For an op-amp with negative feedback, the output is
fed back to the inverting input
A voltage-follower
has a gain of 1, is non-inverting, and has no feedback resistor
A differential amplifier
is part of an op-amp
The use of negative feedback
is to reduces the voltage gain of an op-amp and make linear operation possible
When an op-amp is operated in the single-ended differential mode,
one input is grounded and a signal is applied to the other
In the double-ended differential mode,
only one supply voltage is used
Voltage-follower
A closed-loop, noninverting op-amp with a voltage gain of 1