Class C amplifier
 

I am reading through the excellent book "Solid State Design" by Wes
Hayward et al. I have a specific question on Class C amplifier. On
chapter 2 Fig 15, a Clas C amplifier is shown with a buffer amplifier
link coupled in front.

Why is link coupling needed here, can't the collector be directly
connected to the base? It is for impedance matching or is there any
other motives behind this structure?

73
Ramakrishnan, vu3rdd

rkrishnan wrote:
I am reading through the excellent book "Solid State Design" by Wes
Hayward et al. I have a specific question on Class C amplifier. On
chapter 2 Fig 15, a Clas C amplifier is shown with a buffer amplifier
link coupled in front.

Why is link coupling needed here, can't the collector be directly
connected to the base? It is for impedance matching or is there any
other motives behind this structure?

It's for impedance matching. As usual, there are several ways it can be
done, each with its own advantages and disadvantages. That method is
probably the simplest for the particular application, but you have to
watch the flux density in the transformer primary if it's wound on a
ferrite core with no air gap (such as a toroid core) and make sure the
D.C. current won't cause core saturation. Other methods allow you to
separate the RF choke from the transformer, like in Fig. 16, so you can
optimize the RF choke for its job and the transformer for its job. Or
you can use several types of LC network to do the job.

Roy Lewallen, W7EL

Roy Lewallen wrote:
It's for impedance matching. As usual, there are several ways it can be
done, each with its own advantages and disadvantages. That method is
probably the simplest for the particular application, but you have to
watch the flux density in the transformer primary if it's wound on a
ferrite core with no air gap (such as a toroid core) and make sure the
D.C. current won't cause core saturation. Other methods allow you to
separate the RF choke from the transformer, like in Fig. 16, so you can
optimize the RF choke for its job and the transformer for its job. Or
you can use several types of LC network to do the job.

Roy,

Thanks a lot for the comments. I hope to homebrew some of those early
projects in SSD and then move to bigger projects elsewhere, as I have
not much of hands-on experience in building.

Thanks
Ramakrishnan

rkrishnan wrote:

I am reading through the excellent book "Solid State Design" by Wes
Hayward et al. I have a specific question on Class C amplifier. On
chapter 2 Fig 15, a Clas C amplifier is shown with a buffer amplifier
link coupled in front.

Why is link coupling needed here, can't the collector be directly
connected to the base? It is for impedance matching or is there any
other motives behind this structure?

73
Ramakrishnan, vu3rdd

Any coupling circuit to a class C bipolar transistor amplifier not only
needs to match impedance, it also needs to supply DC current to the base
of the class C stage. Inductive coupling is nice because the average
voltage at the base is nailed at zero and the inductor will guarantee
that enough DC current flows. If you should capacitively couple then
you need to load the base of the final stage with a back-biased diode or
a resistor to provide the current -- otherwise your coupling capacitor
will just charge up until no current flows into your final's base and
you no longer get amplification.

--
-------------------------------------------
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Tim Wescott wrote:
Any coupling circuit to a class C bipolar transistor amplifier not only
needs to match impedance, it also needs to supply DC current to the base
of the class C stage. Inductive coupling is nice because the average
voltage at the base is nailed at zero and the inductor will guarantee
that enough DC current flows. If you should capacitively couple then
you need to load the base of the final stage with a back-biased diode or
a resistor to provide the current -- otherwise your coupling capacitor
will just charge up until no current flows into your final's base and
you no longer get amplification.

Thanks Tim for the info.

I am still a homebrew newbie and am slowly trying to understand the
circuits
described in SSD. Have long forgotten the circuits which I learnt, back
in
my undergrad days..

Is there any good *theory* text which explains everything?

73
Ramakrishnan, VU3RDD