Good afternoon,
Ive come across a problem that has to have a simple solution.
This will be to long to read. Skip to the end for the question.
Hopefully I can properly explain the situation
i think the best way to explain what im trying to do is to start from day 1.
im trying to design a power amplifier for rf applications.
Specs
Input DAC or external function generator. To be determined.
Frequency range 50K~200MHz
Class AB
+47dB output (50watts @ 50 ohms)
My first draft started with a center taped transformer to provide 2 phases of my signal,
then off a common emitter amplifier.
But simulations quickly ruled out the wide frequencie range because of gain losses from the transformer.
My gut says I should stay away from reactive components (capacitors, inductors) as much as possible.
The next draft led me to a phase amplifer, which I have high hopes for.
So I wrote the circuit, taking the input, ac coupled it into the class a common emitter, bumped it up to 20v, then ac coupled into a phase amplifier, then each output ac coupled into its own emitter follower for some current.
After some impedance matching for the stages, I was ready for the power fets.
Except my signals where now out of phase. Stupid caps.
So i played around adjusting some impedances ratios, I tried bringing the transistor to its max Ic, to lower its output inpedance, tried using the emmitor followers to load the phase amp to use them in parallel with the load/emitter resistor.
Basicly tried low,med,high for each stage and every combination and permutation.
i could only remove the problem for small sections of my desired bandwidth.
When I get a phase match at this freq, others are off.
And this is with ideal components ...
So revision # 2 for the phase amplifier
No caps inbetween stages.
So I caught up some of Behzad Razavi's lectures (dude is good, go watch it, the best 90 hours on youtube)
Got the math fresh in my head, and opened excell. And got it looking good.
But now I have an inverted signal, just waiting for amplification, but its at a 9v dc offset.
...... rant over. Safe to start reading again
After some searching in here, w2aew posted a video about the Vbe multiplier circuit
Very interesting bias, seams perfect. Lower a dc voltage just like a diode, 1 transistor, 2 resistors. No phase shiffting.
I love it.
Except i have not been able to get it to work properly.
I can get the dc down, but its acting just like a voltage divider, the more I drop the dc offset,
The more attinuated the signal becomes.
In dropping the dc offset from 10v to 2, the inverted signal amplitude decreases proportionaly
Quick overview of the circuit.
Phase amplifier- r1,r2,r3,r4,q1
Emitter follower for non inverted output - q3,r5
Emittor follower for the inverted output - Q2 and the Vbe multiplier makes up its resistor.
First question. Does a proper Vbe multiplier cause signal attinuation?
Second question. What I screw up?
If i cant get this to work like I want, my plan B is to use a resistor divider to drop the dc offset down,
then add the extra amplification needed to restore the signal in the next stage.
Simulation can be seen here
https://everycircuit.com/circuit/6582741080211456Pic attached
Thank you for your time