The purpose of RF driver

I have got a transmitter schematic in which the carrier frequency producing part is something like this:

Carrier Oscillator->Frequency Divider->RF Driver->Transformer->Power MOSFET Amplifier

I do not understand the purpose of RF driver in the circuit...please help

An "RF driver" is just an amplifier. In order to reach the desired output power for the system, more than one gain stage is probably required. The power MOSFET amp likely doesn't have sufficient gain, alone.

enjunear said:

An "RF driver" is just an amplifier. In order to reach the desired output power for the system, more than one gain stage is probably required. The power MOSFET amp likely doesn't have sufficient gain, alone.

Click to expand...

Agreed, and sometimes you may have a pre-driver stage in there too.
It is common to spread gain out over several stages ( this applies to transmitters and receivers) as it produces better system stability

cheers
Dave

And the transformer is probably being used for isolation purpose only right?

partly isolation ( buffering) between stages and it may also be doing some impedance matching
particularly if the driver is a bipolar device that is then driving the power MOSFET

Dave

Thanks...Yes the driver is indeed a bipolar driving the power MOSFET. However, I was just reading the manual now and there it is written "The RF driver buffers the signal and produces a suitable square wave signal at the carrier frequency capable of driving the power amplifier".

The RF driver shown is basically two bipolar transistors in push pull configuration. I am confused, can a push pull configuration work with square waves? IN that case the driver would be working more as a switch rather than an amplifier...wont it?

raiya_23 said:

Thanks...Yes the driver is indeed a bipolar driving the power MOSFET. However, I was just reading the manual now and there it is written "The RF driver buffers the signal and produces a suitable square wave signal at the carrier frequency capable of driving the power amplifier".

The RF driver shown is basically two bipolar transistors in push pull configuration. I am confused, can a push pull configuration work with square waves? IN that case the driver would be working more as a switch rather than an amplifier...wont it?

Click to expand...

What you described is the basic concept behind a Class D amplifier. Depending on the mode of operation (current or voltage) the desired waveform (typically voltage) is switched between Vsupply and Ground. This generates a lot of harmonic content (squarewave = large odd harmonics), but results in high efficiency at the amplifier because very little power is dissipated by the transistors (only during the switch transitions from high to low, or vice versa).

The tricky part to class D is making them broadband, due to the large amount of post-filtering required to get rid of the (undesired) harmonic frequencies at the output. For a narrowband system, class D amps are quite good at making things more efficient, since you can often make one low-pass filter for the entire TX frequency range and be done.