rf power amplifier

RF – Power Amplifier

Class A

NI Multisim 10 tutorial

Design of a Class A amplifier

Open Multisim 10 and start building the amplifier circuit like in figure.

Press CTRL+W to open the Component Library.

Input signalBiasing circuit

RF amplifier stage

LC resonator AND

impedance matching

circuit

Load


Search Component Library for devices you need to build the circuit.

Use P123 transistor in RF-MOS family.

Analog family contains R, C and L

Sources family is for V and I generators


After you placed all components you need to calculate their parameters in order for the amplifier to work properly.

Let’s go though some mathematic.


POWER TRANSFERAssume we want to build a 1W-RF amplifier.

This meas that the power transferred to the load must be 1 W.

So we must guaratee a maximum value of output impedence of 12.5 Ω. Let’s say we use 10 Ω (Rs).

5.1212

5

22

22

max

2

max P

VddR

R

VddP


OUTPUT MATCHING NETWORKWe use a LC “parallel filter” with Q=10.For parallel circuits,

From here, L and C value can be evaluated as:

pHL 8010102

56//

10// XR

XQ

5,5 CLCL XXXXQ

pFC 2,3101052

16//


LARGE INDUCTOR (BFL)BFL inductor works as a RF choke, because it

“chokes off” the flow of RF current through it, and must be “large enough”.

Large enough means at least 10 times the output impedance (Rs).

HBFLRsX BFL

6.110102

10010

6


DC BLOCKING CAPACITORWe must provide a DC blocking capacitor and

an impedance-transforming network. This function may be combined in a LC series circuit with Q=3.4.

Since , evaluate L and C value as

nHR

Qf

RL loadload 235

4.3101022 61

pHRsQf

Cout 470104.310102

1

2

16

X

RQseries


BIAS POINTClass A amplifier bias point must be choosen

in order to have a conduction angle of 360°.

We use R1=3k and R2=10k. This will give us a bias voltage of

VddVV biasgs

VVddRR

RVbias 85,35

13

10

21

2


Double click on a components placed to edit their value.

“Label” sheet allows you to change the component name.

On “Display” sheet you can change view options.


Set signal generator parameters:Voltage (RMS) = 400mVFrequency (F) = 10 MHzLeave other settings unchanged.

Set Vdd to 5V. Do the same for all remaining components

as depicted in schematic.


Now we have designed a circuit like the one in figure.

We can combine the 2 parallel inductors to have a more compact schematic.


nHLL

LLL 60

21

21//


Place “probes” into circuit to measure voltages and currents.

Probe


Reverse output probe direction by right clicking on the probe arrow and choose “reverse” option.


AC ANALISYS Let’s go through the

AC analisys to understand how well we characterized our amplifier.


Set parameters like in figure.


And, again, select output Voltage (should be Probe3) as the variable to analize.


Output graphs show the amplifier is matched to work at 10 MHz. In fact, amplitude response is unitary and phase is 0.


TRANSIENT ANALISYS Now, analyze

amplifier’s behaviour and output signal using transient analysis.


Set up preferences for transient.

Set final time to 2µs Add output variables:

Id (probe 1) Vin (probe 2) Vout (probe 3)


Class B


Design of a Class B amplifier

BIAS POINTNow, change class A amplifier circuit to build

a new one, but class B. Like for class A, class B amplifier are still linear.

Class B amplifier bias point must be set in order to have a conduction angle of 180°. Changes are in the bias net: Vgs ≈ Vth.


For P123 RF transistor, Vth is about 2V, hence we can change bias circuit resistor’s values to R1=R2=10k.

This means that transistor amplifies only the positive half of the period of the input signal while is off for the rest of time. Complete output waveform is reconstructed by the output LC filter that works as a resonator circuit.

VVddRR

RVbias 5,25

10

10

21

2


AC ANALISYS Again we can analyze

whether the circuit is characterized to work at 10 MHz or not.


You should obtain a frequency response like this one.

As for class A, also class B - RF amplifier is matched to work at 10 MHz.


TRANSIENT ANALISYS


Class C


Design of a Class C amplifier

Class C are non-linear amplifiers. Bias point for this devices is set to have a conduction angle of less than 180°.AC input generator can be replaced by a

pulse generator. Output waveform still will be a sine due to LC oscillator.


Class C amplifier works with Vgs ≈ 0. This implies a set of new circuit changes:No bias network is required. Input signal voltage must be higher than Vth

in order to be able to turn on transistor.


In order to work this circuit needs to be tuned a little better than class A and B amplifier.

We must change some parametres on output stage in order to improove LC resonator and power efficiency.Cout = 200pFC2 = 4.5nF


Double click on the pulse generator to edit its parameters like in figure.


AC ANALISYS


TRANSIENT ANALISYS


AC behaviuor shows again that amplifier is developed for 10 MHz.

Transient analysis demonstrate that class C amplifier do not amplify voltage since output waveform is quite lower than input. In fact, it amplifies current:Analize input and output current orAnalize input and output power


POWER ANALISYS Use wattmeters to measure input and output power.


Double click on wattmeter to see the power value supplied in that point.

Run simulation to see instant power consumed.


POWER GAIN

EFFICIENCY

842.0101.24

103.203

3

DC

out

P

P

5,45103.446

103.206

3

out

in

P

PG

rf power amplifier

Documents

dc blocking capacitor

rf power amplifierclass

amplifier bias point

bias point

output waveform

output power

ac analisys

transient analisys