Multistage And Power Amplifiers - Carleton University

Multistage and Power Amplifiers Compared to single stage amplifier, multistage amplifiers provideincreased input resistance, reduced output resistance, increased gain,and increased power handling capability Multistage amplifiers commonly implemented on integrated circuitswhere large numbers of transistors with common (matched)parameters are available Typical inverter (Common Emitter) has moderately large gain and hasinput and output resistances in the Kilohm range Follower configuration has much higher input resistance, lower outputresistance but has only unity gain Amplifier requires the desirable features of both configurations

vIN vgrIN 0.83 vgrIN RgvOUT av vINRL 4.0 vgRL rOUT

Two Port Amplifier Cascade Impact of input and output loading can be minimized by cascadingtwo amplifiers

Multistage cascading is used to create amplifiers with high inputresistance, low output resistance and large gains

Multistage Amplifier Biasing It is possible to create multistage cascade where each stage isseparately biased and coupled to adjacent stages via DC blockingcapacitors

Amplifier gain is reduced at low frequencies Difficult to build integrated circuits with large value blockingcapacitors DC coupled amplifier

Small Signal Model of Cascaded AmpliferInverterIf R RTHLooking into the emitter of Q3rIN1 rπ 1 rπ 3 2 rπ 1rOUT 1 RC1a1 β ο 1 RC1rπ 1 rπ 3 gm1 RC12(Midband Gain) Assuming matched devices and ro of Q1, Q2 and Q3is large enough to be ignored

Follower (Stage 2)rIN 2 rπ 2 ( β ο 2 1 ) RErOUT 2 REa2 rOUT 1 rπ 2β ο 2 1(β ο 2 1) RErπ 2 ( β ο 2 1) RE(Without R L Connected)

Including this factor in the gain expression (12.23) yields the overallamplifier gain when the load resistor RL is connected:

The amplifier gain can also be derived from the complete small signalmodel

Example:

DC Level Shifting In DC - coupled multistage cascade the output bias level of each stageincreases to maintain the collector more positive than the base(constant current operation) If this voltage “stacking” is severe, little swing room is left in the finalstages of the cascade

Voltage stacking can be alleviated by the use of DC level shiftingLevel shifting alters the bias distribution but not the gainSimple method involves the insertion of a passive device with a constant DCvoltage drop

By using complementary devices, active level shifting can becombined with amplification

In MOSFET circuits, DC level shifting is implemented usingcomplementary NMOS and PMOS devices

Differential Cascade

Power Amplification Stages In many designs an amplifier is required to deliver large amounts ofpower to a passive load. The power may be a large current to a smallresistance or a large voltage to a moderate resistance (impedance) Using a linear amplifier the power wasted in the active device iscomparable to the power delivered to the load. Devices in the outputstage must be capable of dissipating this excess power Alternative configurations offer increased efficiency at the expense oftrue linear operationComplementary Pair (Class B) Output When an amplifier is required to deliver large load currents it isdesirable to bias the voltage of its output terminal near ground. Thisminimizes the bias power dissipated in both the load element and theactive devices of the output stage

Linearly Biased (Class A) Output

Minimally Biased (Class AB) Output

Example:For the following circuit find the bias currents in Q1 and Q2 whenIO 200 µA, IEO 0.8 x 10 -11 mA, R1 40 kΩ.Assume base currents are negligible.

AssumeIEX VBEY 0.6 VVBEY 0.6 V 15 µ AR140 kIEY 185 µ AVBEY η VT lnI EY185 µ A (1)(0.025) ln0.597 V 11I EO0.8 x 10 mAVBEX η VT lnI EX 0.534 VI EOVBE1 VBE 2 VBEX VBEY 0.565 V2I E1vB E I2 I EO e /ηVT 0.565 V 1 (0.8 x 10 11 mA) exp 53 µ A 0.025 V

Integrated Circuit Power Amplifiers A high power device can safely dissipate the heat generated by a largeamount of electrical power Power amplifiers typically use such high powered devices Opamp combined with power amp forms high power opamp whichcan be connected in usual opamp feedback configurations If components are all fabricated on a single chip result is “Integratedcircuit power amplifier” Advantage of integrated power amp is reduced size and simplicity ofuse

Example:Consider figure 12.23 with vOUT connected to v and v IN connectedto v find the output current iO of the low power opamp A 1 as afunction of the output voltage vO . Assume VEE VCC .Solution:Circuit as connected will function as a voltage follower.v vOUT v v INiO i 2 i 1iO(vO VF ) VEE VCC (vO VF ) β F2 R2β F1 R1VEE VCC , R 1 R 2 , βF1 βF2

iO 2 vOβ F 1 R1Example: If βF 20, vOUT 5V , RLOAD 50 Ω EstimateiOUT and iOSolution:iOUT vOUT5V 100 mARLOAD 50 ΩvO vOUTiO Why ?2 vO2 (5V ) 0.33 mAβ F R 1 (20) (15. kΩ )Class AB power amp current gain is 300

Power Devices When large amounts of power are delivered to an amplifier load some powerwill always be dissipated in the transistors of the amplifierPower amplifier stage must use specially fabricated power devices capable ofsafely handling the electrical power dissipated as heatTypical device has a large surface area and is mounted in good thermalambient surroundings

Heat Sinks High power devices are often mounted on metal heat sinks whichenhance the overall thermal contact and increase the removal of heatfrom the devices

A given heat sink is characterized by the heattransfer coefficient or thermal resistance θ whichdescribes the flow of heat from the sink to ambientair for a given rise in heat sink temperatureThe flow of heat is expressed as energ;y flow perunit time and has units of wattsPOUT TSINK TAIRθTo improve thermal conduction, heat sinks may becoated with a thermally conductive compoundIf the overall thermal resistance between thetransistor case and the heat sink is designed asθCASE SINKPOUT TCASE TAIRθ CASE SINK θ SINK AIR

The temperature of the semiconductor device willbe higher than the casePOUT TDEVICE TAIRθ DEVICE CASE θ CASE SINK θ SINK AIRIn thermal equilibrium the electrical powerdissipated will equal the heat flow. We can thendetermine the operating temperature of the device

Example:A power BJT for which θDEVICE - CASE 4 o C / W is mounted on a heatsink with θSINK - AIR 5 o C / W. The mounting uses a 0.2 mm thickmica spacer which introduces an additional thermal resistance of1 o C / W between the transistor case and heat sink. If the BJT carriesan average current (iC ) of 1 A at an average voltage of vCE 10 V.Determine the operating temperature of the semiconductor, transistorcase and heat sink. Assume TAIR 25 o C. Neglect power dissipatedin the base emitter junction of the BJT.

Solution:PELEC iC v CE (1 A)(10 V ) 10 WSince PELEC POUTIn thermal equilibriumTDEVICE TAIR POUT (θDEVICE CASE θ C S θ S A() 25 oC (10 W ) 4 oC / W 1 oC / W 5 oC / W 125 oCSimilarlyTCASE TAIR POUT (θC S θ S A) TSINK TAIR POUT θ S A 75 oC85 oC)

Power BJT

“Star” shaped pattern of transistor increasessurface area of base collector junction where mostof the power is dissipatedManufacturer usually specifies a maximumtemperature Tj-MAX at which the device can beoperated without causing permanent damagePELEC MAX Tj MAX TCASE θ DEVICE CASEA plot of PELEC-MAX versus TCASE is called a powerderating curve of the transistorThe rated power of the device is the maximumelectrical power when the case temperature is equalto the room temperature

The maximum safe operating power hyperbola is defined for the BJTby the relationPELEC-MAX iC vCE Note that there are other limiting specs. (e.g. IC-MAX, BVCEO) whichmay be more stringent than the power operating specs.

Power MOSFETs Power MOSFETs are also capable of dissipating large amounts ofpower The planar MOSFET geometry is no longer suitable due to avalanchebreakdown in the short channel between drain and source Common approach is to use a double diffused vertical MOSFET orDMOS transistor

Short Circuit Protection and Thermal Shutdown The figure below shows a class AB output stage equipped withprotection against the effects of short circuiting the output while thestage is sourcing current

In addition to short circuit protection most IC power amplifiers areusually equipped with a circuit that shuts the amplifier down if a safetemperature is exceeded

Feedback and Stability An opamp that incorporates negative feedback can be represented bythe following block diagram

The “amplifier” block multiplies the voltage vx byAo which represents the open loop gain of theopampThe output of the amplifier is fed into the feedbackblock where it is multiplied by β. The feedbackblock represents the components that form thefeedback networkThe output of the feedback block is subtractedfrom vIN and the result is fed back into theamplifier block as vxvOUT Ao v x Ao (v IN β vOUT ) Ao 1vOUT v IN vIN 1 Aβ 1 β o Ao The factor multiplying vIN is called the closed loopgain. In the limit Ao 1vOUT v IN1β

The output of the amplifier is fed into the feedback block where it is multiplied by β. The feedback block represents the components that form the feedback network The output of the feedback block is subtracted from vIN and the result is fed back into the amplifier block as vx v A v A v v v v A A v A OUT o x o IN OUT OUT IN o o IN o