[Amps] *** SPAM *** Re: CB Amps or full spectrum???

[Peter Chadwick]

Tom said:
>A simple pi network can easily make a class AB2 amp -55dB or more for 
harmonics.<
The second harmonic component of the plate current is -6dB in a class AB amplifier. 
See Pappenfus et al.
That leaves a lot of attenuation to be obtained from an N=3 low pass filter.....
Of course, if you start running below the point where the conduction angle is 180 degrees, things get better, i.e. you've backed off a bit from maximum power.
73
Peter G3RZP






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Message Received: Oct 11 2007, 01:08 AM
From: "Tom Rauch" 
To: g3rzp at g3rzp.wanadoo.co.uk, amps at contesting.com
Cc: 
Subject: *** SPAM *** Re: [Amps] CB Amps or full spectrum???

> Firstly, if you do the sums for a Class AB tube amplifier 
> with the usual Q values, you'll see that you don't meet 
> either the FCC requirements for harmonics (46dB) nor the 
> Radio Regulations requirements, which are 4 dB tighter at 
> 50dB.>>

You keep saying that Peter, but it isn't true. A simple pi 
network can easily make a class AB2 amp -55dB or more for 
harmonics.

For example the AL-12 series of amps makes about -60dBc on 
bands where it is a pi-net. It really isn't significantly 
better on bands where it is a pi-L.

>> Which is why the manufacturers these days use a Pi-L 
>> network.>>

Not that I know. I use a pi-L to extend matching range.

In the world of REAL antennas and real feedlines, the L 
section often doesn't do much at all. It can even make 
things worse.

> Biasing the 12 volt transistors is fun anyway. From a DC 
> viewpoint, you'd like a constant current source of bias 
> that reduced as the transistors got warm and the beta went 
> up. But a constant current source is ideally an infinite 
> impedance, and as soon as you apply RF, the rectification 
> then alters the bias point>>

That's right. Many people miss this and think a simple diode 
makes a dandy bias source.

In fact a simple diode almost never works well above a few 
mA of base current. I've measured amps with a shunt diode 
for bias and at full drive the base bias voltage is often 
NEGATIVE by several volts. Unfortunately many people don't 
measure dynamic regulation of screen or bias supplies, they 
just look at zero drive bias current.

73 Tom