There are three other designs ofopamp besides the familiar typewith high input impedance and lowoutput impedance. In this articleUlrich Tietze (co-author of a well-known book on circuit design) andThomas Ußmüller find out whatthese other types can offer interms of bandwidth with the helpof a few essential circuits.

34

KNOW-HOW AMPLIFIERS

Voltage mode or

In many applicationsordinary opamps areoutperformed bytheir more exoticcousins

Von Thomas Ußmüller and Ulrich Tietze

Photograph: courtesy Analog Devices

How to choose an

35

What would electronics be like without operationalamplifiers? Almost every electronic device that uses ana-logue circuitry will contain one or more of these compo-nents. It is hard to believe that the first opamps were con-structed from valves: as in so many things, developmentswere led by military requirements, for example in appli-cations such as controlling the aim of guns during thewar. The first commercial opamps did not come onto themarket until 1952. The valve opamp designed byGeorge A. Philbrick Researches was fitted with an octalsocket (see photograph).

Shortly thereafter the first opamps made from discretetransistors were produced. The first integrated opampwas the µA702, designed for Fairchild by RobertWidlar. It was first available in 1962. Six yearslater came the µA741, which has maintained ahold on the market ever since. These importantdevices are now produced in their millions, andthey are used in applications ranging from audioand video amplifiers to active filters and sensorsignal conditioners.

There are four basic types of operationalamplifier: the device can have a high or alow input impedance inverting input and ahigh or a low output impedance output (seeFigure 1). The most widely-used type,familiar to practically all electronics engi-neers, is the voltage-mode, or ‘VV’ type.This has a high input impedance and alow output impedance (the non-invertinginput has a high impedance on alldevice variants). In many circuits, how-ever, one of the alternative types,called ‘CC’ (current-mode), ‘VC’ (volt-age-to-current, or transconductance)or ‘CV’ (current-to-voltage, or tran-simpedance) amplifiers, can offer asignificantly higher bandwidth. Weshall now examine this claim inmore detail with the help of a cou-ple of simple circuits.

Let’s build an opampFirst of all we shall show how the four types

of operational amplifier can be constructed frombasic blocks. To make the comparison as fair as pos-sible we shall use the same components in each case.We decided to use a technique, common in CV- andCC-type operational amplifiers, called push-pull classAB (also known as ‘current on demand’) operation.

current mode?

IoUo

IoUo

Io = k I = S UI N D DUo = I Z = A UN D D

Io = S UD DUo = A UD D

ININ

UDUD

UDUD

current amplifier(C-C opamp)

transimpedance amplifier(C-V opamp)

transconductance amplifier(V-C opamp)

regular opamp(V-V opamp)

voltage output

volta

ge

inp

ut

current output

cu

rre

nt

inp

ut

UN UP

1Uo

1

high-impedance junction

frequency correction

1

++

++

––

––

2 rS

050151 - 12

–

–

T'1T'2

T1

T2

+

I0

I0

+

T4T3

++

a voltage follower

b current mirror

050151 - 13

Figure 1.The four different typesof operational amplifiershown schematically.

Figure 2.In class AB push-pulloperation the voltage-mode amplifier usesonly two different basicblocks: a voltagefollower and a currentmirror.

Figure 3.Internal circuit of thevoltage follower andcurrent mirror.

opamp

As we can see from Figure 2, there are only two basiccircuits that we need: a voltage follower and a currentmirror. The innards of these circuits are shown inFigure 3.

The two complementary voltage followers form the neces-sary differential amplifier at the input to the opamp. Thesignal is taken in the form of a current drawn from thepower supply from the voltage followers to the currentmirrors. The advantage of push-pull class AB operationwhen compared to a conventional differential amplifier isthat, despite the low quiescent current, almost arbitrarilylarge peak currents can be obtained. A consequence ofthis is that there is no slew rate limit. The output of thetwo current mirrors is the high impedance point of the cir-cuit. If necessary, a capacitor (called a ‘compensationcapacitor’) may be connected to ground from this point.The final part of the operational amplifier circuit is a volt-age follower which provides a low impedance output.

Voltage mode under the microscope...A voltage-mode operational amplifier is characterised byhigh-impedance inputs and a low impedance output. Itoperates as a voltage-controlled voltage source. Theideal amplifier exhibits a gain of

AD = Uo / UD = ∞ (infinity).

Real opamps have a differential gain of between 103

and 106. From the model of a real amplifier (Figure 4)we can see that the differential gain is determined by thetransconductance resistances rS = 1/S in the impedanceconverter at the input and by the impedance at the highimpedance point.To investigate its bandwidth the operational amplifier isconnected as shown in Figure 5. The feedback resistorshave values RN = R1 = 100 Ω. This gives the circuit again of

A = 1 + RN / R1 = 1 + 100 Ω / 100 Ω = 2.

Without a compensation capacitor we get considerableovershoot in the step response of this amplifier, and sowe connect a compensation capacitor to the high imped-ance point. This succeeds in suppressing overshoot but,unfortunately, also reduces the bandwidth of the circuit.The frequency response of the circuit is shown inFigure 8.

... and current modeA current-mode operational amplifier has a low-imped-ance inverting input and a high-impedance output.Exactly as with other types of operational amplifier, thenon-inverting input has a high impedance. The model isshown in Figure 6. Another common name for this typeof amplifier is ‘diamond transistor’, which derives fromthe similarity of its behaviour to that of an ideal bipolartransistor. Commercially-available current-mode opera-tional amplifiers include the MAX435 and MAX436 fromMaxim. The well-known OPA660 from Burr-Brown issadly no longer produced, but comprehensive documen-tation is available on the Internet: see the links below.To use a current-mode operational amplifier as a non-inverting amplifier, it can be connected in the same wayas a voltage-mode opamp with negative voltage feed-back: see Figure 7. This circuit arrangement is also

elektor electronics- 11/200536

KNOW-HOW AMPLIFIERS

ZUoIq

rS

rS

UD

+

–

1 1

1

050151 - 14

ZIoscIq

rS

UD

+

–

1

UN UPUa

1

B

C

E

++

a model

b equivalent diagram

c circuit symbol

– –

rS

050151 - 16

Ui UD

Uo

R1

RN

050151 - 15

a with opamp symbol b with transistor symbol

IC

IC

= IE

= IE

IE 2IE

IEUo

UoUi Ui

Ue Ue

RN

RN

R1R1

050151 - 17

Figure 4.Model of a real volt-

age-mode operationalamplifier. The differen-tial gain is determined

by resistors rs in theimpedance converter at

the input and by theimpedance at the high-

impedance point.

Figure 5.Connecting a voltage-

mode operationalamplifier for a

bandwidth test (RN = R1 = 100 Ω).

Figure 6.Model of a

current-modeoperational amplifier.

Figure 7.A current-mode

operational amplifierused as a non-invertingamplifier with negative

voltage feedback(‘direct feedback’):

RN = 200 Ω, R1 = 100 Ω.

called ‘direct feedback’. In our simulation we set RN =200 Ω and R1 = 100 Ω. The gain is again given by

A = 1 + RN / 2 R1 = 1 + 200 Ω / (2 × 100 Ω) = 2.

The cutoff frequency of the voltage amplifier can bedetermined from Figure 8.

An alternative arrangement for constructing a non-invert-ing amplifier from a current-mode opamp uses theopamp’s ideal transistor properties. It can be connectedin an emitter-follower configuration with negative currentfeedback (Figure 9). Here the resistor values are RC =200 Ω and RE =100 Ω. The gain is then

A = RC / RE = 200 Ω / 100 Ω = 2.

This arrangement is termed ‘current feedback’. The band-width is similar to that achieved using negative voltagefeedback (see Figure 8).As well as the amplifiers we have described here, wehave also simulated a VC opamp and a CV opamp,which we can construct from the same basic blocks.Again, we examined the bandwidth of each circuit; wehave collected the results together in the Table. Thereare only small differences in bandwidth between the vari-ous amplifier configurations. Voltage-mode and VCopamps require a capacitor to correct the frequencyresponse, thus ensuring stable operation of the amplifier.

Which is the best opamp to use in an integrator?An important operational amplifier circuit is the integra-tor. It finds practical application in, for example, filters(‘biquads’). Figure 10 shows the circuit of an integratorusing a voltage-mode operational amplifier, which canoperate at a frequency of up to 2 MHz. Above that fre-quency is a region where the gain increases again (seeFigure 11). The reason for this behaviour can be seenfrom the equivalent circuit (Figure 12). Capacitors Cand C1 are effectively a short-circuit to high frequencies.A voltage divider is thus formed by R and ro, giving anoutput voltage of

Uo = ro / (R + ro) Ui.

The current-mode operational amplifier can be connectedas an integrator as shown in Figure 13. As the fre-quency response clearly shows, the bandwidth of this cir-cuit is much greater than that of the voltage-mode integra-tor. The upper frequency limit is around 24 MHz (see Fig-ure 11). Also, there is no frequency range over whichthis configuration exhibits differentiating behaviour. Thisis because the parasitic capacitance at the high imped-ance point of the current-mode operational amplifier isconnected in parallel with the integrator capacitor anddoes not cause a parasitic ‘pole’ on the frequencyresponse.In summary we can say that the current-mode operationalamplifier is much better suited to use in integrator circuitsthan the voltage-mode opamp: firstly, its bandwidth issome ten times greater, and secondly, it does not exhibitdifferentiating behaviour at higher frequencies.

And finally: a filterFinally we shall look at the frequency response of a sec-ond-order Butterworth low-pass filter. The filter was

11/2005 - elektor electronics 37

1.0kHz- 5

0

5

10

10kHz 100kHz 1.0MHz 10MHz

050151 - 18

100MHz 1.0GHz

[dB]

V-V opamp (with correction capacitance)-V opamp (with correction capacitance)C-C opamp (direct fC-C opamp (direct feedbaceedback)k)

C-C opamp (current fC-C opamp (current feedbaceedback)

BC

E

Ui

IE

RE

IE

RE UoRC

IE

UoUi Ue RE RC

IE

Ue RE

050151 - 19

Uo

R

C

Ui

UD

050151 - 20

a model b high frequencies

1Uo UoUi Ui ro

ro

C

R1 C1 U1

R R

UD SUD

V-V opamp

050151 - 22

1.0Hz- 120

- 80

0

- 40

40

100Hz10Hz 10kHz 100kHz1.0kHz 1.0MHz 10MHz

050151 - 21

100MHz 1.0GHz

[dB]

V-V opamp-V opamp C-C opampC-C opamp

Figure 8.Frequency response ofthe amplifiers. Shownare a voltage-modeopamp with compensa-tion capacitor (green), acurrent-mode opampwith direct feedback(red) and a current-mode opamp with cur-rent feedback (blue).

Figure 9.A current-mode opampconnected as an idealtransistor in anemitter-followerconfiguration withnegative currentfeedback: RC = 200 Ω,RE =100 Ω.

Figure 10.Circuit of an integratorusing a voltage-modeoperational amplifier.

Figure 11.Characteristic curves ofintegrators realisedusing voltage-mode(green) and current-mode (red) opamps.The current-mode circuitis clearly superior.

Figure 12.Equivalent circuit of anintegrator constructedfrom a voltage-modeopamp. Capacitors Cand C1 are effectivelyshort circuits at highfrequencies.

realised using voltage-mode integrators (see circuit inFigure 14) and with current-mode integrators (see cir-cuit in Figure 15). An ideal filter was used as a bench-mark. The cutoff frequency chosen was fc = 3 MHz. Look-ing at the frequency responses (Figure 16) we can seethat the filter using voltage-mode integrators (green curve)soon departs from the ideal characteristic (blue curve). Atfrequencies above 30 MHz the filter is no longer usable.The integrator filter using current-mode operational ampli-fiers (red curve) follows the ideal characteristic moreclosely than the filter using voltage-mode integrators; it issuitable for use with frequencies up to 100 MHz.

ConclusionThe conclusion is that a ‘normal’ voltage-mode opera-tional amplifier is adequate for most applications. In par-ticular, when used as an amplifier, the differencebetween the types is not great as long as the same circuitconfiguration is used. In integrators and integrator filters,however, current-mode operational amplifiers outperformvoltage-mode opamps in respect of bandwidth by a fac-tor of around ten.

(050151-1)

Bandwith comparison

Amplifiers based on different opamps

Amplifier Bandwidth

VV 32.1 MHz

VC 49.4 MHz

CV 59.1 MHz

CC (direct feedback) 58.1 MHz

CC (current feedback) 29.5 MHz

elektor electronics- 11/200538

KNOW-HOW AMPLIFIERS

1

IC = I E

IE

IE

IE UoUoUi

UiUe

rS ro Co CC

R

R

050151 - 23

Figure 13. Circuit and model of an

integrator using acurrent-mode

operational amplifier.

Ui

R2

R

R

R1

Uo

R

C

C

R

050151 - 24

Figure 14.Second-order Butter-worth low-pass filter

realised using voltage-mode integrators.

UTP

UiUBP

T1 T2

T3

R1 RR

CC

050151 - 25

Figure 15.Second-order Butter-worth low-pass filter

realised using current-mode integrators.

1.0kHz- 100

- 50

0

10kHz 100kHz 1.0MHz 10MHz

050151 - 26

100MHz 1.0GHz

[dB]

V-V opamp-V opamp C-C opampC-C opamp ideal filterideal filter

Figure 16.Characteristic curves of

the low-pass filters.The integrator filterusing current-mode

operational amplifiers(red) follows the ideal

characteristic (blue)more closely than the

voltage-mode integra-tor filter (green), and is

suitable for use withfrequencies of up to

100 MHz.

ReferenceTietze, U. and Schenk, C.: Electronic Circuits: Handbook for Design and Application,Springer, ISBN 3-540-50608-X / 0-387-50608-X

Web linkswww.eetimes.com/anniversary/designclassics/opamp.html

http://en.wikipedia.org/wiki/Operational_amplifier

www.uoguelph.ca/~antoon/gadgets/741/741.html

http://focus.ti.com/lit/an/sboa071/sboa071.pdf