[snip]
An RC high pass divider (signal into capacitor, output between cap and
resistor, resistor grounded) will produce 45 degrees of phase lead at
the frequency where the capacitive reactance equals the resistance.
Mathematically, R=1/(2*pi*f*c).
However, the output amplitude will be 70.7% of the input amplitude.
If you make a pole zero phase lead all pass filter with an opamp, not
only will the amplitude be independent of frequency, you can tune the
phase shift without altering the gain.
This sort of thing is made by adding an opamp and two equal resistors
to the phase lead divider I mentioned above. You connect the output
pf that divider to the + input of the opamp, and connect the divider
made up of the two equal resistors from input signal to the output of
the opamp, with the middle tap connected to the - input of the opamp.
The resistor after the cap can be an adjustable resistor to trim the
phase shift.
Are you 'Trolling' here John?
I did a posting here last year on the subject of an all-pass
phase-lead circuit called 'The all-pass conundrum'.
Basically, it seems it can't be done. No matter what configuration of
R/C or L/C you use, you get an output voltage vector that rotates
clockwise with increasing frequency, so the output phase progressively
lags as frequency increases.
The only way, (in theory), to get what you want is if either the R or
the C is negative, but the combination of negative resistance and
capacitance, or resistance and negative capacitance, always results in
either oscillation or latch-up.
However, the all-pass phase-lag circuit gives us a means of generating
phase lag, without the normally attendant attenuation so, by analogy,
I've felt for a long time now that we aught to be able to do the
converse - i.e. phase lead without gain.
The trite answer you invariably get when discussing an all-pass
phase-lead is that it would amount to a time-machine. Whilst I can
see what they mean, I've never entirely bought that myself. The fact
is, we can and do generate phase-lead, without the need for the use of
a Tardis, albeit accompanied by increasing gain with increasing
frequency, with all the problems that invariably brings. So, if we
can generate phase-lag, without attenuation, why can't we generate
phase-lead without gain?
All I can say is that I've come at the problem from all sorts of
different angles and I've never been able to find a solution.
Regards
Ted Wilson