[Amps] Direct rectification of AC mains to drive the amp, VDD Supply

[Manfred Mornhinweg]

Bill,

> That's doing it the hard way. Just set the scope input to AC instead of DC.
> Problem solved. 

I wish it were that easy. But it isn't. For example, let's consider this 
case: You want to check the gate drive voltage, with an amplifier that's 
floating with the AC line, while your scope is grounded.

So you set up the scope to display the difference between channels, then 
connect one channel to the amplifier's common (which is NOT at ground), 
and the other channel to the gates. The shields of both probes get 
joined, but not connected anywhere else.

Result: Depending exactly how the power supply is configured, and how 
your supply is grounded, the scope channel connected to the amp's common 
can see anything from zero up to the full 240V RMS swing, at 50 or 60Hz. 
The other channels sees that same, plus the gate drive voltage.

The gate drive will only be a few volts, so you might want to set up the 
scope for 1V per division, after considering the probe's attenuation. 
With 10:1 probes, you set the scope to 0.1 V/div.

That WOULD display the clean, nice, pure gate drive voltage, if and only 
if the following conditions are met:

- Each channel can cope with the full 240V swing plus gate voltage, 
without saturating or even loosing linearity. That would be about 65V 
p-p, through the 10:1 probe. It's asking a bit too much, when the scope 
is set for 0.8V p-p full scale!

- The gain and phase of the two channels are matched well enough to 
really cancel out the strong 50 or 60Hz signal. This part is actually 
much easier to get well.

The vertical amplifiers of my scope, a Tektronix from the late 1970's, 
saturate at about 3 times the voltage required for full deflection. I 
could use this scope to measure the drain signals by the channel 
difference method, without problems, but not the gate voltage. To keep 
it from saturating, I could set the vertical gain only to 40V full scale
(5V/div), with 10:1 probes. The next setting, 2V/div, would already 
saturate. And at 5V/div and 10:1 in the probes, my gate signal might be 
0.05 divisions, essentially the same as the width of the trace!

So, it does not work for that purpose. A special transformer-coupled RF 
probe would be required, or one of the other techniques.

> And always use at least a 10X probe for RF measurements. A 1X probe has too
> much capacitance.  

Yes. And not only that - also a simple 1:1 probe has a termination 
problem. The cable might be 75 ohm or a little more, while the scope 
input is 1 megaohm in parallel with some capacitance. You get standing 
waves on the cable, resulting in totally wrong amplitude on screen! So, 
even when one is measuring a very low impedance source, for which the 
high capacitance of 1:1 probes is no problem, teh measurement will most 
likely be wrong. 1:1 probes are useful only at low frequencies, where 
the cable length is a negligible part of the wavelength.

On the other hand, a good way to go is a simple 1:1 probe made just from 
a piuece of 50 Ohm coax, with a 50 Ohm termination resistor right in the 
BNC plug. VHF and UHF scopes, and most spectrum analyzers, usually work 
this way, and can even have the 50 Ohm loads built in.

Of course, don't connect such a probe to a high impedance point! ;-)

Well, back to my radio now. I'm having some fun today building a "world 
receiver", an all-band AM receiver optimized for that mode, that covers 
exactly the AM broadcast bands - all of them. It's a fun project, 
combining old and new technology - the IF amp uses dual gate MOSFETs 
biased negative by a silver oxide cell, in the style radios from the 
1930's did with tubes and mercury batteries, while the local oscillator 
is a 14 bit direct digital synthesizer driven from a 400MHz clock. 
Tuning is by optical encoder, and the thing has 100 memories stored in 
EEROM.
I think I will build a wooden cabinet for it, shaped like a cathedral 
radio from the early 30's.

I just had to tell somebody! ;-)

Manfred

========================
Visit my hobby homepage!
http://ludens.cl
========================