Ch12.ppt

Chapter 12Chapter 12Operational AmplifiersOperational Amplifiers

ObjectivesObjectives Describe basic op-amp characteristics

Discuss op-amp modes and parameters

Explain negative feedback

Analyze inverting, noninverting, voltage follower, and inverting amp configurations Describe the impedance characteristics of the three op-amp configurations

Discuss op-amp compensation

Troubleshoot op-amps

Introduction To Operational AmplifiersIntroduction To Operational Amplifiers

The operational amplifier or op-amp is a circuit of components integrated into one chip. We will study the op-amp as a singular device. A typical op-amp is powered by two dc voltages and has an inverting(-) and a noninverting input (+) and an output. Note that for simplicity the power terminals are not shown but understood to exist.


While an ideal op-amp has infinite gain and bandwidth, we know this is impossible. However, op-amps do have very high gain, very high input impedance, very low output impedance, and wide bandwidth.


It is interesting that the op-amp is internally made up of circuits that we have discussed in previous chapters.

Figure 12–4Figure 12–4 The basic differential amplifier. The basic differential amplifier.

Thomas L. Floyd Thomas L. Floyd Electronic Devices, 7eElectronic Devices, 7e

Copyright ©2005 by Pearson Education, Inc.Copyright ©2005 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458Upper Saddle River, New Jersey 07458

All rights reserved.All rights reserved.

Figure 12–5 Basic operation of a differential amplifier showing Figure 12–5 Basic operation of a differential amplifier showing the effect on the currents when a small voltage is connected to one the effect on the currents when a small voltage is connected to one

of the bases.of the bases.


In single ended input mode one input is grounded.


With the differential input mode two out-of-phase signals are applied with the difference of the two amplified and produced at the output.


With common mode input, two signals of same phase, frequency, and amplitude are applied to the inputs which results in no output. This is called common-mode rejection. This type of mode is used for removal of unwanted noise signals.


The common-mode rejection ratio (CMRR) is the measure for how well it rejects an unwanted the signal. It is the ratio of open loop gain (Aol) to common-mode gain (Acm). The open loop gain is a data sheet value.

CMRR = Aol /Acm


Op-amps tend to produce a small dc voltage called output error voltage (VOUT(error)). The data sheet provides the value of dc differential voltage needed to force the output to exactly zero volts. This is called the input offset voltage (VOS). This can change with temperature and the input offset drift is a parameter given on the data sheet.

Introduction To Operational AmplifiersIntroduction To Operational AmplifiersThere are other input parameters to be considered for op-amp operation. The input bias current is the dc current required to properly operate the first stage within the op-amp.

The input impedance is another. Also, the input offset current—which can become a problem if both dc input currents are not the same.

Output impedance and slew rate—the response time of the output with a given pulse input—are two other parameters.

Op-amp low frequency response is all the way down to dc. The high frequency response is limited by the internal capacitances within the op-amp stages.

Negative FeedbackNegative Feedback

Negative feedback is feeding part of the output back to the input to limit the overall gain. This is used to make the gain more realistic so that the op-amp is not driven into saturation. Remember that regardless of gain, there are limitations of the amount of voltage that an amplifier can produce.

Op-Amps With Negative FeedbackOp-Amps With Negative Feedback

The closed-loop voltage gain (Acl) is the voltage gain of an op-amp with external feedback. The gain can be controlled by external component values. Closed loop gain for a noninverting amplifier can be determined by the formula below.

A cl(NI) = 1 + Rf/R1


The voltage-follower amplifier configuration has all of the output signal fed back to the inverting input. The voltage gain is 1. This makes it useful as a buffer amp since it has a high input impedance and low output impedance.


The inverting amplifier has the output fed back to the inverting input for gain control. The gain for the inverting op-amp can be determined by the formula below.

A cl(I) = Rf/R1

Effects Of Negative Feedback Effects Of Negative Feedback On Op-Amp ImpedancesOn Op-Amp Impedances

However high the input impedance of an op-amp circuit is, impedance still exists. For a noninverting amplifier it can be determined by the formulas below.

B(feedback attenuation) = Ri/Ri + Rf

Zin(NI) = (1 + AolB)Zin

Effects Of Negative Feedback On Effects Of Negative Feedback On Op-Amp ImpedancesOp-Amp Impedances

The output impedance is understood to be low for an op-amp. Its exact value can be determined by the formula below.

Z(out) = Zout/1 + AolB

Effects Of Negative Feedback On Effects Of Negative Feedback On Op-Amp ImpedancesOp-Amp Impedances

The input impedance for an inverting amplifier is approximately equal to the input resistor (Ri).

The output impedance is very low and in most cases any impedance load can be connected to it with no problem. The exact amount can be determined by the formulas below.

B(feedback attenuation) = Ri/Ri + Rf

Zout(I) = Zout / (1 + AolB)

Bias Current And Offset Voltage Bias Current And Offset Voltage CompensationCompensation

Input bias current creates an output error voltage that must be compensated for in all of the op-amp configurations. For the voltage-follower this error voltage can be reduced with resistors of the same value in the feedback loop and input.


For the inverting and noninverting configurations this can be accomplished by a resistor in the noninverting part of the circuit that has the same value as the feedback resistor.


Input offset voltage is small but unavoidable because of internal characteristic differences. The offset null terminals and a potentiometer connected as shown can eliminate this.

Open-Loop ResponseOpen-Loop Response

The open-loop gain of an op-amp is determined by the internal design and it very high. The high frequency cutoff frequency of an open-loop op-amp is about 10 Hz.


The internal RC circuit of an op-amp limits the gain at frequencies higher than the cutoff frequency. The gain of an open-loop op-amp can be determined at any frequency by the formula below.

Aol = Aol(mid)/1 + f 2/fc2

Op-amp with internal RC circuit shown externally.


Of course, as with any RC circuit, phase shift begins to occur at higher frequencies. Remember that we are viewing internal characteristics as external components.

Closed-Loop ResponseClosed-Loop Response

Op-amps are normally used in a closed-loop configuration with negative feedback. While the gain is reduced the bandwidth is increased. The bandwidth (BW) of a closed-loop op-amp can be determined by the formula below. Remember B is the feedback attenuation.

BWcl = BWol(1 + BAol(mid))

Closed-Loop ResponseClosed-Loop Response

The gain-bandwidth product is always equal to the frequency at which the op-amp’s open-loop gain is 0 dB (unity-gain bandwidth).

TroubleshootingTroubleshooting

You can only troubleshoot an op-amp as a single device even though it can be quite complex internally. Let’s look at some common failures of the op-amp and the effects of associated external component failure.


With an open feedback resistor (Rf) in a noninverting amplifier, the op-amp’s gain goes up to the open loop gain. The result would be severe clipping of the output signal.


With an open input resistor (Ri) the gain would drop to approximately 1.

Other signal degradation would be indicative of internal op-amp failure.

TroubleshootingTroubleshootingAn open feedback resistor in an inverting amplifier would have similar effects as a noninverting amplifier. An open input resistor would prevent the input signal from reaching the amp.

Internal op-amp failures would be the main suspect with a voltage follower aside from external connection problems.

SummarySummary

The basic op-amp has three terminals: inverting input (-), noninverting (+), and output.

An op-amp has a very high open-loop gain, very high input impedance, very lower output impedance, and wide bandwidth.

Input offset voltage and current produce an error voltage at the output. These can be compensated for by external components. Gain can be controlled by a resistive feedback loop.

Common-mode occurs when equal in-phase voltages are applied to both terminals, which results in no output.

SummarySummary

The bandwidth of an op-amp is equal to the upper critical frequency (fcu).

The bandwidth increases as the gain is decreased. The internal RC circuits are responsible for high frequency roll-off.

The gain-bandwidth product equals the frequency at which unity voltage gain occurs.

The three op-amp amplifier configurations are inverting, noninverting, and voltage follower.