2 Outcome of Chapter 4Ability to perform simple DESIGN and EVALUATE A, B and AB classes of BJT and FET amplifiers, in terms of their frequency response, equivalent circuit, termal management (power dissipation) and gain.
3 Outline Introduction Concept of Power Amplifier Power Amplifier ClassificationBJTs/ MOSFETs Power AmplifierClass A Power Amplifier
4 IntroductionPower amplifiers are used to deliver a relatively high amount of power, usually to a low resistance load.Typical load values range from 300W (for transmission antennas) to 8W (for audio speaker).Although these load values do not cover every possibility, they do illustrate the fact that power amplifiers usually drive low- resistance loads.Typical output power rating of a power amplifier will be 1W or higher.Ideal power amplifier will deliver 100% of the power it draws from the supply to load. In practice, this can never occur.The reason for this is the fact that the components in the amplifier will all dissipate some of the power that is being drawn form the supply.
5 Concept of Power Amplifier Provide sufficient power to an output load to drive other power device.To deliver a large current to a small load resistance e.g. audio speaker;To deliver a large voltage to a large load resistance e.g. switching power supply;To provide a low output resistance in order to avoid loss of gain and to maintain linearity (to minimize harmonic distortion)To deliver power to the load efficiently
6 Power Amplifier Power Dissipation The total amount of power being dissipated by the amplifier, Ptot , isPtot = P1 + P2 + PC + PT + PEThe difference between this total value and the total power being drawn from the supply is the power that actually goes to the load – i.e. output power.
7 Power Amplifier Efficiency A figure of merit for the power amplifier is its efficiency, h .Efficiency ( h ) of an amplifier is defined as the ratio of ac output power (power delivered to load) to dc input power .By formula :As we will see, certain amplifier configurations have much higher efficiency ratings than others.This is primary consideration when deciding which type of power amplifier to use for a specific application.
8 Power Amplifiers Classification Class A - The transistor conducts during the whole cycle of sinusoidal input signalClass B - The transistor conducts during one-half cycle of input signalClass AB - The transistor conducts for slightly more than half a cycle of input signalClass C - The transistor conducts for less than half a cycle of input signal
9 Maximum Theoretical Efficiency, max Efficiency RatingsThe maximum theoretical efficiency ratings of class-A, B, and C amplifiers are:AmplifierMaximum Theoretical Efficiency, maxClass A25%Class B78.5%Class C99%
10 BJT Power AmplifierComparison of the characteristics and maximum ratings of a small-signal and power BJTParameterSmall-signal BJT(2N2222A)Power BJT(2N3055)(2N6078)VCE (max) (V)4060250IC (max) (A)0.8157PD (max) (W)1.21154535 – 1005 – 2012 – 70fT (MHz)3001
11 Typical dc beta characteristics (hFE versus Ic) for 2N3055
12 BJTs Power Amplifier Current gain is smaller in power amplifier BJT. The gain depends on IC and temperature may be related to the following:maximum current that connecting wires can handleat which current gain falls below a stated valuecurrent which leads to maximum power dissipation.maximum voltage limitation associated with avalanche breakdown in reverse-biased collector-base junction.second breakdown in BJT operating at high voltage and current.
13 VCE(sus) =115 volt at which these curve merge and the minimum voltage necessary to sustain the transistor in breakdown.The breakdown voltage, VCE0 ~130 volt when the base terminal is open circuited, IB=0
14 Instantaneous power dissipation The average power over one cycleThe maximum rated power,
15 MOSFETs Power Amplifier Characteristic of two power MOSFETsParameterPower MOSFET2N67572N6792VDS (max) (V)150400ID (max) (A) (at T = 25C)82PD (max) (W)7520
16 Performance Characteristic of MOSFETs Power Amplifier Faster switching timesno second breakdown.Stable gain and response over wide temperature range.
17 Class A Amplifieroutput waveform same shape input waveform + phase shift.The collector current is nonzero 100% of the time. inefficient, since even with zero input signal, ICQ is nonzero(i.e. transistor dissipates power in the rest, or quiescent, condition)
19 Typical Characteristic Curves for Class A Operation Configuration : No transformer are usedCommon-emitter amplifier,dc load line (the Q point is at centre of the load line)instantaneous power dissipation versus time in the transistor
20 DC Input PowerThe total dc power, Pi(dc) , that an amplifier draws from the power supply :Note that this equation is valid for most amplifier power analyses. We can rewrite for the above equation for the ideal amplifier as
21 AC Output Power AC output (or load) power, Po(ac) Above equations can be used to calculate the maximum possible value of ac load power. HOW??Disadvantage of using class-A amplifiers is the fact that their efficiency ratings are so low, max 25% .Why?? A majority of the power that is drawn from the supply by a class-A amplifier is used up by the amplifier itself.
24 ExampleCalculate the input power [Pi(dc)], output power [Po(ac)], and efficiency [h] of the amplifier circuit for an input voltage that results in a base current of 10mA peak.
25 ExampleThe common source circuit parameters are VDD=10V, RD=5kΩ and the transistor parameters are Kn=1mA/V2, VTN=1V and =0.Assume the output voltage swing is limited to the range between the transition point and vDS=9V to minimize nonlinear distortion.Calculate the actual efficiency of a class A output stage.
26 Exercise The Q-point of common source circuit is VDSQ=4V Find IDQ Determine the max peak to peak amplitude of a symmetrical sinusoidal output voltage if the min value of instantaneous drain current must be no less than 0.1IDQ and the min value of instantaneous drain source voltage must be no less than vDS=1.5V.Calculate the power conversion efficiency where the signal power is the power delivered to RL.Ans:60mA,5V,31.25mW, 5.2%
27 Design of Class A C-E Amplifier To find R1, R2, RE, RC use the DC analysis and design formula;To find Zi, Zo, Av, Ai use AC analysis (without loading effect)To find Zi, Zo, Avs, Ai use AC analysis (with loading effect if have Ri and RL
29 Class B Power Amplifier Consists of complementary pair electronic devicesOne conducts for one half cycle of the input signal and the other conducts for another half of the input signalWhen the input is zero, both devices are off, the bias currents are zero and the output is zero.Ideal voltage gain is unity
30 For input larger than zero, A turn ON and supplies current to the load. For input less than zero, B turn ON and sinks current from the load
44 The maximum possible average signal power delivered to the load The possible average signal power supply by VCCThe maximum possible power conversion efficiency
45 Transformer Coupled Amplifier The theoretical maximum efficiency of a basic RC- coupled class-A amplifier is limited to 25%.In practical circuit, the efficiency is less than 25%.Used for output power of about 1 W only.Transformer coupling can increase the maximum efficiency to 50%Disadvantage of transformer coupling – expensive & bulky.
46 Neglecting transformer resistance and assuming RE is small;
48 Turn ratio is designed for maximum symmetrical swing, hence; The maximum average power delivered to load equals maximum average power delivered to the primary of the transformer(VCC and ICQ are maximum possible amplitudes of signal)
49 The average power supplied by the VCC source is; The maximum possible efficiency is;