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