ME 244L Department of Mechanical Engineering
Dynamic Systems and Controls Lab The University of Texas at Austin

Basic Amplifier Circuits

 Laboratory Note

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Contents

The Operational Amplifier

The operational amplifier (op amp) is used in many electronic circuits to perform linear and nonlinear functions.  Voltage operational amplifiers are distinguished by high voltage gain, high input impedance, and low output impedance, as opposed to transconductance operational amplifiers, which have high input impedance, high output impedance, and a current output linearly proportional to input voltage.  It is common to refer to voltage op amps simply as op amps.  Op amps were originally developed for use in analog computing.

Basic Op Amp

The symbol for a basic op amp is shown below.   Note that the op amp does not have a ground terminal, but all voltages are referenced to ground through the power supplies.

Operational amplifier (op amp) and voltages reference to ground.

Gain. A = -vout/vab.  This "open loop" gain is expressed in terms of the differential input voltage, vab.  The gain is a function of frequency, being highest at "d.c." (zero frequency) and rolling off at high frequency at 20 dB/dec.  The manufacturer's specifications will give more detail.

Input Resistance. The input resistance, Ri, is the resistance between the inverting and non-inverting input terminals.  Typically these values range from kiloohms to megaohms for op amps.

Output Resistance.  The output resistance, Ro, of op amps range from a few tens of ohms to hundreds of ohms.  The output resistance will affect how much of the "ideal" output voltage under "no load" is actually applied to a load when a current flows.  That is, by referring to the typical Thevenin type model, the actual output voltage will be,

vout = vout(open) - Ro*iload,

where vout(open) is the open-circuit output voltage and iload is the current flowing to the load.

Common-Mode Gain (CMG) and Common Mode Rejection (CMR).  Common mode signals applied to both the input terminals are typically rejected because their difference will be zero.  However, practical effects cause some common mode effects to be amplified by the common mode gain.  Op amps are designed to have a low CMG.  The common mode rejection ratio (CMRR)  is the absolute value of the ratio of the differential gain divided by the common mode gain.

More detailed performance specifications. (a link will be placed here later.
 

The 741 Op Amp

The 741 op amp is commonly used for general purpose amplifier circuits.  The basic


 

Typical input resistance for a 741 ranges from 300 kiloohms to 2 megaohms.

Basic Comparator

See this note from Mims, "Timer, Op Amp and Optoelectronic Circuits and Projects", Radio-Shack, 2000:

Basic Comparator (pdf)

Voltage Amplifier Circuits

Inverting and non-inverting amplifiers are relatively easy to build using an op amp IC.  There are a few tests that might be conducted after any of these circuits are assembled.
  • Check for offset voltage due to imperfections in the device.  There are offset pins on IC op amps that allow you to construct offset adjusting circuits.  Another way to eleminate offset is to build a summing circuit with one input an adjustable voltage that can be used to cancel the offset.
  • Check the actual gain achieved in the final circuit; this value may vary from what is expected because of tolerances in the components.  Use both dc and ac signals if time permits.
  • Use the gain and the device data sheets to estimate the bandwidth of the final amplifier circuit.  This is a bit more subtle
Inverting Amplifier

The inverting amplifier shown below is one of the most basic and can be configured from a single op amp.  The output voltage, however, is inverted: vout = -(R2/R1)*vin.

 

Input resistance: Ri = R1.

Output resistance: Ro = Ro*(1+R2/R1)/A.
 

Non-Inverting Amplifier

The non-inverting amplifier shown below has an output voltage: vout = (1+R2/R1)*vin.  So the minimum gain is 1 in the case that R2 is made zero.  The input resistance is

Rin = (Ri*A)/(1+R2/R1).


 
Choosing between Inverting and Non-inverting configurations

  • If a gain less than unity is needed, the inverting amplifier must be used.
  • If a very high input resistance is needed, the noninverting amplifier should be used.  The input resistance in this case is the product of the amplifier gain and Ri (see above).  The input resistance of the inverting amplifier is equal to the resistance between the signal and the inverting terminal.  It is not always possible to choose very large value of this resistance because the overall design can lead to larger than desired noise and bias currents.
  • If multiple signals are to be input to an amplifier (as for a summer circuit, see below), then it is more convenient to use the inverting amplifier, because each signal independently contributes to the output.  It can be shown that in a noninverting configuration, there is some interdependence between the signals.

The Differential Amplifier

The differential amplifier accepts signals at the inverting and the noninverting inputs as shown below.  This circuit will reject common mode signals that originate at the source, or from other sources in the system (e.g., ground loop signlas, noise from a transducer, etc.).

Offset Adjustment

Off-the-shelf op amps will have an offset voltage, which is defined as that differential voltage that must be applied to the input terminals to bring the output voltage to zero.  The input offset voltage can range up to a few millivolts in magnitude.
 
Op amps will often have offset null pins, and a potentiometer can be placed across these terminals to apply an adjustable voltage that will bring the output to zero when there is no input.

Another way to bring down an offset is to use multiple signals "summed" at the input, as in a summing circuit (see below).

The Voltage Follower

In a voltage follower, the output voltage is equal to the input voltage.  The voltage follower is used for isolation or impedance transformation.  It is often used as a buffer between a source (e.g., a generator, sensor, etc.) and a load (e.g., a motor, a meter, etc.).
 

Other Linear Applications

Summing Amplifier

This circuit allows you to add multiple signals.  Two back to back op amps can be used so the signal will be inverted again in a second stage.

This circuit can also be used as shown for a single input where the input at 2 has a high dc bias.  The input at 1 could be adjusted by a potentiometer to bring the output voltage to zero (by cancelling the bias).

Integrator

Differentiator

Instrumentation Amplifier

Bridge Amplifiers

Current-to-voltage converter

Photosensor amplifier

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Send comments to: Prof R.G. Longoria, Department of Mechanical Engineering, The University of Texas at Austin at r.longoria@mail.utexas.edu  . Last updated October 21, 2005 .