7.CLASSES OF RF AMPLIFIERS

RF. AMPLIFIERS.
Classes of amplifiers class A, B, AB, C, D, E and F
Class A
This circuit permits a simultaneous large voltage and current swing at the output of the device.
The circuit will always be on in this kind of amplifier unless some other switching circuit is added. Biasing is at the midway where current swing will be I max/2
Power efficiency is always 50% or less and cannot go beyond that amount.
Temperature is also another concern that the device may not be overheated.

iD = IQ + I peak cos not
vD = Vcc + I PEAK RL cos lot
PRF = I PEAK 2 RL
2
Effi = I PEAK 2 RL and PDISS = Pdc - PRF
2 IQ Vcc
The conductor angle of class A amplifier is 360o or 2 radians.
It is a larger gain compared to the rest of amplifiers
CLASS – B amplifier.
With a conductor angle of 180o or 3.14radians the circuit is independent of drive level.
When Vin exceeds a threshold value (Vo) the output current (Io) = G (Vin – Vo) and when it falls below threshold, Io = O
If G is trasnconductance and Vo is the bias voltage
Vin = Vb + Vicos wt
When the amplifier is on the output current mirrors he peaks of the inoput sinusoidal voltage .
The peak of the sine wave tips of the Io is just the trasconductance times the amount by which the input volatage exceeds the threshold that is
Ip=(Vi – Vx) G
Io(t) =IP/3.14 + IP/2 coswt +2Ip/3Pie cos 2wt – (2Ip/15pie) cos 4w + ----
Pdc = VccIp/pie
PRF = VPEAK IPEAK/2
This efficiency is close to pie/4 or 78% when the peak value of the output element half-sinusoid if achieves its maximum value Imax.
Its major advantage is its increased efficiency thus allowing considerable improvement in radio talk-time. That is when Ic = 0 for half cycle the output voltage is the tightest.
Disadvantages is its little gain
Its biasing is as follows: I(q) is set close to zero. The drain can be kept the same as for class A operation, although if the bias point is selected so that VD is allowed to swing up to breakdown voltage at high levels, it must be reduced by Vp/2 compared to class -A operation. This is because the breakdown occurs between drain and the gate; as the gate voltage is reduced form - Vp/2 in class A to –Vp for lass-B; this pulls the drain- gate diode closer to break down.
The drain voltage is therefore reduced by the same amount to keep the differential voltage across the diode the same. As long as the drain is biased through an RF choke, the voltage at drain can “float” around its average bias value of VDD. The endpoints of class – B wave form are the same as those of class – A . Its average slope and optimum resistors are the same too. Currents and voltage are the same too.

Push-pull configuration
]in class-B amplifier, the harmonic components of the output current are short-circuited by a tank circuit at the load.
In microwave and RF power devices the tank circuit is often omitted because the harmonics tend to be short circuited by the device output capacitance.
Some distortion is taken in exchange of improved efficiency.

Class-AB amplifier
Defined by a conduction angle that lies between 180o and 360o and the device is switched off for a portion of a cycle when the input voltage swings sufficiently into cut off. It conducts somewhere between 50% and 100% of a cycle depending on the bias levels chosen. As a result, its efficiency and linearity are intermediate between class A and B amplifier. It is a positive current bias rather than negative.

Class – C amplifier (switching mode amplifier)
The gate bias is arranged to cause the transistor to conduct less than half the time. It is traditional to approximate these pulses by the top pieces of sinusoids to facilitate a direct analysis.


iD= IDC + irf sinwot, iD>o where the offset IDC which is analogous to bias current in a linear amplifier is actually negative for a class C amplifier. The overall drain current iD is always positive or zero. Here the transistor behaves at all times as a current source (high output impedance).
Max Efficiency =(2X-Sin2X)/4(SinX-XCosX)
This result to a very close to 100% efficiency as the conduction angle shrinks towards zero.

Class D Amplifiers
An active device controlled by current source is used as a switch since switch dissipates no power and therefore 100% efficiency theoretically. Like push-pull class B amplifier, the input connection guarantees that only one transistor is driven on at a given time with one transistor handling the positive half-cycles and the other the negative half cycles, but driven hard enough to make the transistor act like a switch.
Each primary terminal of the output transforms (T2) is alternately driven to ground yielding a square –wave voltage across the primary (and therefore across the secondary) winding. When one drain goes to zero volts, transformer action forces the other drain to a voltage of 2 Vdd. The output filter allows only the fundamental components of this square wave to flow into the load.
Disadvantages are that the inclusion of a transformer makes the device bulky. And only functions well only at frequencies below transformer frequency.

CLASS E AMPLIFIERS
Though similar to class B or F amplifiers due to its 180o conduction angle, between saturation and cutoff, It uses switching principle and thus loosing its analog relationship between the input and output.
Modeled as a switch in parallel with the device output capacitance, that can be supplemented by an additional shunt capacitance. Total drain or collector current is then alternately steered between the device saturation resistance (i.e. the closed or on switch).
The device output current source is assumed Zero during the off half-cycle.
With 50% a duty cycle, extrinsic drain load at the fundamental frequency that provides the class-E wave-forms is inductive. And is independent of both the input and level and drain supply voltage. At microwave frequencies, the nonlinear nature of the intrinsic component calculated of this optimum load somewhat difficult.
Therefore ZL =R(1+j1.1525)

CLASS F AMPLIFERS
Here, the output tank is tuned to resonance at the carrier frequency and is assumed to have a high enough Q to act as a shot circuit at all frequencies outside of the desired band width.
The length of the transmission line is chosen to be precisely a quarter – wavelength at the carrier frequency. That is the input impedance of such a line is proportional to the reciprocal of the termination impedance
Zin = Zo2
ZL

The drain sees an open circuit at all even harmonies of the carrier since the transmission line appears as some integer multiple of a half-wavelength at all even harmonics. Conversely, the drain sees and open circuit at all odd harmonics of the carrier because the tank appears as a open circuit. Transmission line appears as an odd multiple of a quarter – wavelength and therefore provides a net reciprocation of the load impedance.

VDS = (4/3.14) 2VAD

Po = [(4/3.14) VDD][4/3.14 X VDD]/ 2R

This power amplifier as a peak to peak value which exceeds the totaT VDS swing due to Fourier transformation.
It has a high efficiency than all, up to 100% it also as a substantially better normalized power-handling capability because the maximum voltage is just twice the supply.


References
The design of CMOS Radio Frequency integrated circuits by Thomas H. Lee and
RF circuit design for modern wireless systems by rowan Gilmore and less bessser