 # POWER AMPLIFIER CHAPTER 4.

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POWER AMPLIFIER CHAPTER 4

Outcome of Chapter 4 Ability 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.

Outline Introduction Concept of Power Amplifier
Power Amplifier Classification BJTs/ MOSFETs Power Amplifier Class A Power Amplifier

Introduction Power 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.

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

Power Amplifier Power Dissipation
The total amount of power being dissipated by the amplifier, Ptot , is Ptot = P1 + P2 + PC + PT + PE The 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.

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.

Power Amplifiers Classification
Class A - The transistor conducts during the whole cycle of sinusoidal input signal Class B - The transistor conducts during one-half cycle of input signal Class AB - The transistor conducts for slightly more than half a cycle of input signal Class C - The transistor conducts for less than half a cycle of input signal

Maximum Theoretical Efficiency, max
Efficiency Ratings The maximum theoretical efficiency ratings of class-A, B, and C amplifiers are: Amplifier Maximum Theoretical Efficiency, max Class A 25% Class B 78.5% Class C 99%

BJT Power Amplifier Comparison of the characteristics and maximum ratings of a small-signal and power BJT Parameter Small-signal BJT (2N2222A) Power BJT (2N3055) (2N6078) VCE (max) (V) 40 60 250 IC (max) (A) 0.8 15 7 PD (max) (W) 1.2 115 45 35 – 100 5 – 20 12 – 70 fT (MHz) 300 1

Typical dc beta characteristics (hFE versus Ic) for 2N3055

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 handle at which current gain falls below a stated value current 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.

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

Instantaneous power dissipation
The average power over one cycle The maximum rated power,

MOSFETs Power Amplifier
Characteristic of two power MOSFETs Parameter Power MOSFET 2N6757 2N6792 VDS (max) (V) 150 400 ID (max) (A) (at T = 25C) 8 2 PD (max) (W) 75 20

Performance Characteristic of MOSFETs Power Amplifier
Faster switching times no second breakdown. Stable gain and response over wide temperature range.

Class A Amplifier output 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)

Basic Operation Common-emitter (voltage-divider) configuration (RC-coupled amplifier)

Typical Characteristic Curves for Class A Operation
Configuration : No transformer are used Common-emitter amplifier, dc load line (the Q point is at centre of the load line) instantaneous power dissipation versus time in the transistor

DC Input Power The 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

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.

Limitation

Example Calculate 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.

Example The 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.

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%

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

Example & Exercise Will be given in our class.

Class B Power Amplifier
Consists of complementary pair electronic devices One conducts for one half cycle of the input signal and the other conducts for another half of the input signal When the input is zero, both devices are off, the bias currents are zero and the output is zero. Ideal voltage gain is unity

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

Complimentary Push-Pull Circuit

The Ideal Class B

Maximum possible value of Vp is VCC
The instantaneous power in Qn is;

The average power in Qn is
Differentiating for maximum PQn with respect to Vp equal to zero gives us Maximum average power dissipation;

The average power delivered to the load is
Power source supplies half sinewave of current, the average value is; The total power supplied by the two sources is

The efficiency is

Class AB Power Amplifier
Small quiescent bias on each output transistor to eliminate crossover distortion

Class C Power Amplifier

Class AB Voltage Transfer Curve

Collector Currents & Output Current

Example The parameters are VDD=10V, RL=20Ω. The transistor are matched and K=0.2A/V2, VT=1V, IDQ=0.05 when vo=5V. Determine the required biasing in a MOSFET class AB output stage.

Inductively Coupled Amplifier

The maximum possible average signal power delivered to the load
The possible average signal power supply by VCC The maximum possible power conversion efficiency

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.

Neglecting transformer resistance and assuming RE is small;

For ideal transformer;

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)

The average power supplied by the VCC source is;
The maximum possible efficiency is;