the resource handbook of electronics, 8353ch 13

7/28/2019 The Resource Handbook of Electronics, 8353CH 13

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Whitaker, Jerry C. Analog Circuits

The Resource Handbook of Electronics.Ed. Jerry C. Whitaker

Boca Raton: CRC Press LLC, 2001

2001 by CRC PRESS LLC


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Chapter

13Analog Circuits

13.1 Introduction

Amplifiers are the functional building blocks of electronic systems, and each of these

building blocks typically contains several amplifier stages coupled together. An am-

plifier may contain its own power supply or require one or more external sources of

power. The active component of each amplifier stage is usually a transistor or an FET.

Other amplifying components, such as vacuum tubes, can also be used in amplifier

circuits if the operating power and/or frequency of the application demands it.

13.2 Single-Stage Transistor/FET Amplifier

The single-stage amplifier can best be described using a single transistor or FET con-

nected as a common-emitteror common-source amplifier, using an npn transistor(Figure 13.1a) or an n-channel FET (Figure 13.1b) and treating pnp transistors or

p-channel FET circuits by simply reversing the current flow and the polarity of the

voltages.

At zero frequency (dc) and at low frequencies, the transistor or FET amplifier stage

requires an input voltage E1

equal to the sum of the input voltages of the device (the

transistorVbe

orFET Vgs

) andthevoltage across theresistanceReorR

sbetweenthe com-

monnode (ground)andthe emitter or sourceterminal. TheinputcurrentI1to theampli-

fier stage is equal to the sum of the current through the external resistor connected be-

tween ground and the baseor gateand the basecurrentIbor gate currentI

gdrawn bythe

device. In most FET circuits, the gate current may be so small that it can be neglected,

while in transistor circuits the base current Ibis equal to the collector current I

cdivided

by the current gain beta of the transistor. The input resistanceR1to theamplifierstage is

equal to the ratio of input voltage E1 to input current I1.The input voltage and the input resistance of an amplifier stage increases as the

value of the emitter or source resistor becomes larger.

The output voltage E2of the amplifier stage, operating without any external load, is

equal to the difference of supply voltage V+ and the product of collector or drain load



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resistorR1and collectorcurrentI

cor draincurrentI

d. Anexternal loadwillcause the de-

vice to draw an additional current I2, which increases the device output current.

As long as thecollector-to-emitter voltage is larger than thesaturation voltage of the

transistor,collectorcurrent willbe nearly independent of supply voltage.Similarly, the

draincurrentof anFET will benearly independentofdrain-to-sourcevoltageas long as

this voltage is greater than an equivalent saturation voltage. This saturation voltage is

approximately equal to the difference between gate-to-source voltage and pinch-off

voltage, the latter being the bias voltage that causes nearly zero drain current. In some

FET data sheets, the pinch-off voltage is referred to as the threshold voltage. At lowersupply voltages, the collector or drain current will become less until it reaches zero,

when the drain-to-source voltage is zero or the collector-to-emitter voltage has a very

small reverse value.

Figure 13.1 Single-stage amplifier circuits: (a) common-emitter NPN, (b) com-mon-source n-channel FET, (c) single-stage with current and voltage feedback.

(a) (b)

(c)



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The output resistance R2

of a transistor or FET amplifier stage isin effectthe

parallel combination of the collector or drain load resistance and the series connectionof two resistors, consisting ofR

eorR

s, and the ratio of collector-to-emitter voltage and

collector current or the equivalent drain-to-source voltage and drain current. In actual

devices, an additional resistor, the relatively large output resistance of the device, is

connected in parallel with the output resistance of the amplifier stage.

The collector current of a single-stage transistor amplifier is equal to the base cur-

rent multiplied by the current gain of the transistor. Because the current gain of a tran-

sistormaybe specified as tightlyas a two-to-one rangeatonevalue ofcollectorcurrent,

or itmay have just a minimum value, knowledgeof theinput currentis usually notquite

sufficient to specify the output current of a transistor.

13.2.1 Impedance and Gain

The input impedance is the ratio of input voltage to input current, and the output im-pedance is the ratio of output voltage to output current. As the input current increases,

the output current into the external output load resistor will increase by the current

amplification factor of the stage. The output voltage will decrease because the in-

creased current flows from the collector or drain voltage supply source into the col-

lector or drain of the device. Therefore, the voltage amplification is a negative num-

ber having the magnitude of the ratio of output voltage change to input voltage

change.

The magnitude of voltage amplification is often calculated as the product of

transconductance Gm

of the device and the load resistance value. This can be done as

long as the emitter or source resistance is zero or the resistor isbypassed with a capaci-

tor that effectively acts as a short circuit for all signalchanges of interest but allows the

desired bias currents to flow through the resistor. In a bipolar transistor, the

transconductance is approximately equal to theemitter current multiplied by39, which

is thechargeofa singleelectrondividedby theproductofBoltzmanns constant andab-

solute temperature in degrees Kelvin. In a field-effect transistor, this value will be less

and usually proportional to the input-bias voltage, with reference to thepinch-off volt-

age.

The power gain of the device is the ratio of output power to input power, often ex-

pressed in decibels.Voltage gain or current gain can be stated in decibels but must be so

marked.

Theresistor in serieswith theemitter or sourcecausesnegative feedback of most of

the output current, which reduces the voltage gain of the single amplifier stage and

raises its input impedance (Figure 13.1c). When this resistorReis bypassed with a ca-

pacitorCe, theamplificationfactorwill be high at high frequencies andwill be reduced

by approximately 3 dB at the frequencywhere the impedanceof capacitorCeis equal to

the emitter or source input impedance of the device, which in turn is approximately

equalto the inverse of thetransconductance Gm

of the device (Figure 13.2a). The gain of

the stage will be approximately 3 dBhigher than the dc gain at the frequency where the

impedance of the capacitor is equal to the emitter or source resistor. These simplifica-



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tions hold in cases where the product of transconductance and resistance values are

much larger than 1.

A portionof the outputvoltagemayalso be fed backto the input,whichis the baseor

gate terminal. This resistorRf

will lower the input impedance of the single amplifier

stage, reduce current amplification, reduce output impedance of the stage, and act as a

supply voltage sourcefor thebase or gate. This methodisused whenthesource of input

signals, and internal resistance Rs, is coupled with a capacitor to the base or gate and a

group of devices with a spread of current gains, transconductances, or pinch-off volt-

ages must operate with similar amplification in the same circuit. If the feedback ele-

ment isalso a capacitorCf, high-frequencycurrentamplificationof thestagewill be re-duced byapproximately 3 dB when the impedance of the capacitor is equal to the feed-

back resistorRfand voltage gain of the stage is high (Figure 13.2b). At still higher fre-

quencies, amplification will decrease at the rate of 6 dB per octave of frequency. It

should be noted that the base-collector or gate-drain capacitance of the device has the

sameeffectof limitinghigh-frequencyamplification of thestage; however, thiscapaci-

tance becomes larger as the collector-base or drain-gate voltage decreases.

Feedback of theoutput voltage through an impedance lowersthe input impedanceof

an amplifier stage. Voltage amplification of the stage will be affected only as this low-

ered input impedance loads the source of input voltage. If the source of input voltage

has a finite source impedanceand the amplifier stage has very high voltage amplifica-

tion andreversed phase,the effectiveamplificationforthisstagewill approach theratio

of feedback impedance to source impedance and also have reversed phase.

13.2.2 Common-Base or Common-Gate Connection

For the common-base or common-gate case, voltage amplification is the same as in

the common-emitter or common-source connection; however, the input impedance is

Figure 13.2 Feedback amplifier voltage gains: (a) current feedback, (b) voltage feed-back.

(a) (b)



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approximately the inverse of the transconductance of the device. (SeeFigure 13.3a.)

As a benefit, high-frequency amplification will be less affected because of the rela-

tively lower emitter-collector or source-drain capacitance and the relatively low inputimpedance. This is the reason why the cascade connection(Figure 13.3b) of a com-

mon-emitter amplifier stage driving a common-base amplifier stage exhibits nearly

the dc amplification of a common-emitter stage with the wide bandwidth of a com-

mon-base stage. Another advantage of the common-base or common-gate amplifier

Figure 13.3 Transistor amplifier circuits: (a) common-base NPN, (b) cascode NPN, (c)common-collector NPN emitter follower, (d) split-load phase inverter.

(a) (b)

(c) (d)



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stage is stable amplification at very high frequencies and ease of matching to RF

transmission-line impedances, usually 50 to 75 .

13.2.3 Common-Collector or Common-Drain Connection

The voltage gain of a transistor or FET is slightly below 1.0 for the common-collector

or common-drain configuration. However, the input impedance of a transistor so con-

nected will be equal to the value of the load impedance multiplied by the current gain

of the device plus the inverse of the transconductance of the device (Figure 13.3c).

Similarly, the output impedance of the stage will be the impedance of the source of

signals divided by the current gain of the transistor plus the inverse of the

transconductance of the device.

When identicalresistors areconnected between thecollectoror drain andthesupplyvoltage andtheemitter or sourceand ground, an increase inbase or gate voltage will re-

sult inan increaseof emitter or sourcevoltagethat isnearly equal to thedecreaseincol-

lector or drain voltage. This type of connection is known as the split-load phase in-

verter, useful for driving push-pull amplifiers, although the output impedances at the

two output terminals are unequal (Figure 13.3d).

Thecurrentgain of a transistor decreasesat high frequencies as theemitter-base ca-

pacitance shunts a portion of the transconductance, therebyreducing current gain until

it reaches a value of 1 at the transition frequency of the transistor(Figure 13.4). From

this figure it can be seen that the output impedance of an emitter-follower or com-

mon-collector stage will increase with frequency, having the effect of an inductive

source impedance when the input source to the stage is resistive. If the source imped-

ance is inductive, as it might be with cascaded-emitter followers, theoutput impedance

of such a combinationcanbe a negative valueat certainhighfrequenciesandbe a possi-ble cause of amplifier oscillation. Similar considerations also apply to common-drain

FET stages.

Figure 13.4 Amplitude-frequency response of a common-emitter or common-sourceamplifier.



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13.2.4 Bias and Large Signals

When large signals have to be handled by a single-stage amplifier, distortion of the

signals introduced by the amplifier itself must be considered. Although feedback can

reduce distortion, it is necessary to ensure that each stage of amplification operates in

a region where normal signals will not cause the amplifier stage to operate with

nearly zero voltage drop across the device or to operate the device with nearly zero

current during any portion of the cycle of the signal. Although described primarily

with respect to a single-device-amplifier stage, the same holds true for any amplifier

stage with multiple devices, except that here at least one device must be able to con-trol current flow in the load without being saturated(nearly zero voltage drop) orcut

off(nearly zero current).

If the single-device-amplifier load consists of the collector or drain load resistor

only, the best operating point should be chosen so that in the absence of a signal,

one-half of thesupply voltage appears as a quiescent voltageacross theload resistorRl.

If an additional resistive loadRlis connectedto theoutput through a coupling capacitor

Cc(Figure 13.5a), the maximum peak load currentI

lin one direction is equal to the dif-

ference between quiescent current IIof the stage and the current that would flow if the

collector resistor and theexternal load resistor wereconnectedin seriesacross thesup-

plyvoltage.In the otherdirection, the maximumload current is limitedby the quiescent

voltage across thedevice divided by theload resistance. Thequiescent current flows in

the absence of an alternating signal and is caused by bias voltage or current only. Be-

cause most audio frequency signals (and others, depending upon the application) havepositive and negative peak excursions of equal probability, it is usually advisable to

have the two peak currents be equal.This canbe accomplishedby increasing thequies-

cent current as the external load resistance decreases.

Figure 13.5 Output load-coupling circuits: (a) ac-coupled, (b) series-parallel ac,push-pull half-bridge, (c) single-ended transformer-coupled.

(a) (b) (c)



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When several devices contribute current into an external load resistor (Figure

13.5b), one useful strategy is to set bias currents so that the sum of alltransconductances remains as constant as practical, which means a design for mini-

mumdistortion.This operatingpoint forone device is near one-quarter thepeak device

currentfor push-pullFETstagesand at a lesservalue forbipolarpush-pullamplifiers.

When the load resistance is coupled to the single-device-amplifier stage with a

transformer(Figure 13.5c), the optimum bias current should be nearly equal to the peak

current that would flow through the load impedance at the transformer with a voltage

drop equal to the supply voltage.

13.3 Operational Amplifiers

An operational amplifier is a circuit (device) with a pair of differential input terminals

that have very high gain to the output for differential signals of opposite phase at each

input and relatively low gain for common-mode signals that have the same phase ateach input (seeFigure 13.6). An external feedback network between the output and

the minus () input and ground or signal sets the circuit gain, with the plus (+) input at

signal or ground level. Most operational amplifiers require a positive and a negative

power supply voltage. One to eight operational amplifiers may be contained on one

substrate mounted in a plastic, ceramic, or hermetically sealed metal-can package.

Operational amplifiers may require external capacitors for circuit stability or may be

internally compensated. Input stages may be f ield-effect transistors for high input im-

pedance or bipolar transistors for low-offset voltage and low-voltage noise. Available

types of operational amplifiers number in the hundreds. Precision operational ampli-

fiers generally have more tightly controlled specifications than general-purpose

types.Table 13.1 details the most common application and their functional parame-

ters.

Figure 13.6 Operational amplifier with unbalanced input and output signals and a fixedlevel of feedback to set the voltage gain V

g, which is equal to (1 + R)/R.



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Table 13.1 Common Op-Amp Circuits (From[1]. Used with permission.)


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The input-bias current of an operational amplifier is the average current drawn by

each of the two inputs, + and , from the input and feedback circuits. Any difference indc resistance between the circuits seen by the two inputs multiplied by the input-bias

current will be amplified by the circuit gain and become an output-offset voltage. The

input-offset currentis the difference in bias current drawn by the two inputs, which,

when multiplied by the sum of the total dc resistance in the input and feedback circuits

and thecircuit gain, becomes an additional output-offset voltage.The input-offsetvolt-

age is the internal difference in bias voltage within the operational amplifier, which,

when multiplied by thecircuit gain, becomes an additional output-offset voltage. If the

normal input voltage is zero, theopen-circuit outputvoltage is thesumof thethree off-

set voltages.

13.4 References

1. Whitaker, Jerry C. (ed.), The Electronics Handbook, CRC Press, Boca Raton, FL,1996.

13.5 Bibliography

Benson, K. Blair (ed.), Audio Engineering Handbook, McGraw-Hill, New York, NY,1988.

Fink, Donald (ed.),Electronics Engineers Handbook, McGraw-Hill, NewYork, NY,1982.

Whitaker, Jerry C.,and K. Blair Benson(eds.), Standard Handbook of VideoandTele-vision Engineering, 3rd ed., McGraw-Hill, New York, NY, 2000.

Whitaker, Jerry C. (ed.), Video and Television Engineers Field Manual,McGraw-Hill, New York, NY, 2000.

END

the resource handbook of electronics, 8353ch 13

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