The basic circuit of RF power amplifier consist of the RF power transistor, RF choke and input and output network. The transistor may be BJT, MOSFET or MESFET type.

Below is the basic RF power amplifier configuration with MOSFET transistor.



Basically, the transistor may be operated as a dependent current source or a switch or combination of these.

When operated as dependent current source the circuit looks the following.

Here the drain current ID is determined by the gate to source voltage VGS and the transistor operating point. The drain voltage VD is determined by the drain current ID and the load impedance. In order to operate the transistor as current dependent source, it must be operated in the active region(pinch off region or saturation region). It should not enter the ohmic region. For this reason, the drain to source voltage VDS must be kept higher than a minimum VDSmin , that is VDS > VDSmin = VGS-VT where VT is the transistor threshold voltage. In such case, there is linearity between IDS and VDS w.r.t VGS. Hence such design is useful for linear power amplifier.

When the transistor is operated as switch the block diagram looks like the following.

In this case, when the switch if OFF, the switch current ID is zero and the swtich voltage VDS is determined by the external output network circuit response. When the switch if ON, the switch voltage VDS is nearly zero and the drain current or the switch current is determined by the external circuit.

In order to operate the MOSFET transistor as a switch, the transistor should not enter the active region or pinch off region. It must remain in the ohmic region when the switch is ON and in the cutoff region when the switch if OFF. To make MOSFET operating in the ohmic region the draing  to source voltage VDS must must less than the VGS-VT, that is, VDS < VGS-VT.

Besides these basic transistor operation, there is the output network and the input network.

The main functions of the output network(and input network) is to provide a impedance matching, suppress harmonic distortion and filtering the output signal to limit them within a bandwidth in order to avoid interference with adjacent circuit network.

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