For sure not the first time for me visiting this awesome site, it saved me quite a few times - this time, however, I have a specific question ... would be great if someone could help me with this!
I need a software-tunable oscillator with frequencies from 100 kHz to 2 MHz and an amplitude of at least 10 V(pp). So the -3dB bandwidth should be high enough to have a stable gain between 100k-2MHz. I realized that this is already quite a challenge. Well, I decided to go for the famous AD9850 which I successfully hooked up on an Arduino and it runs like a charm! However, the amplitude of the signal is only 1 Vpp, so I do need an amplifier. This is where it's hard for me to wrap my head around.
So far, I used OpAmps a lot. Here, this doesn't appear to be the best approach since a gain of at least 10 with 2 MHz bandwidth and 10 V(pp) sounds like I need an expensive OpAmp with both high GBP and insane slew rate ... So I decided to go for an common emitter amplifier! I read quite a few articles, but as a hobbyist - hell that's confusing stuff! However, I got a circuit running and the simulation looked quite promising as you can see below - and I am sure, those are not necessarily the best values to choose. I used a BC547C I found around and built the circuit ... and was quite disappointed when I hooked up the AnalogDiscovery2's network-analyser.
Unfortunately, I don't have an image of the Bode-plot anymore. Aafter digging deeper in the internet I figured that the collector-base parasitic capacitance, about 9 pF for the BC547C, is killing me - when inserting this capacitance (C3 in the image below) I get the EXACT same curve as measured with the AnalogDiscovery2 in the real circuit.
Well, crap! There is no way to get around this capacitance here ... again I went for a search on the internet and found some "HF"-labelled transistors with high GBP and (promised) low capacitance-values for C(cb). Now, here are some more details about the circuit's intended use:
- Impedance of the (filtered) AD9850's output is 200 Ohms (at least that's what I designed the filter for and that works quite well...)
- Output is going to be hooked to an high impedance conductivity sensing circuit with at least 200 kOhm input impedance (which is R6 here, 1 MOhm is just an educated guess, could even be more...)
- ... so effectively I need an VOLTAGE amplifier, not so much of an power amplifier.
- I have a voltage source for up to 35 V as supply - this should not be a problem (above this is V2 with 20V here)
- I do have some "HF" transistors/JFETs here, including the SC1730, BF545A, BF959, BFS17 an NTE312 and a BFW92 that do all have significantly lower C(cb) values
I would really appreciate some advice on:
- if the use of one of these transistors/JFETs or - more generally - any transistor/JFET with lower C(cb) and GBP can fix my problem, or if I am running in a totally wrong direction here
- Would you recommend more than one gain stage for a stable 20dB gain between 100 kHz and 2 MHz?
- and would it even be possible to go beyond +20dB?
Just one last thing: I would like to keep the cost of the circuit as low as possible, which is why I reckon a CE-amplifier (or something like this) would be a good solution since transistors/JFETs are cheap.
Thanks for all your answers, great to get such nice help! Here a few things I'd like to share:
I looked it up, for sure C(cb) was included in the BC547's LTspice model - stupid me. Although I did made some substantial progress in understanding impedance, clearly I am bad in spotting it in action. The AD2's input impedance with its 25 pF as pointed out by some of you seems to be a major problem.
I rebuild the model posted by Bruce below with parts I had laying around, I've got a picture of the LTspice circuit:
And the actual thing:
I follwed the general advice and soldered it to a perfboard to minimise parasitic capacitance - and well, it works great! (although looking a bit like a bungling...). Below I got the actual Bode plot from the simulation and of the measurement with my AD2:
Since it's hard to see: I've got 25.3 dB @ 100 kHz, 25.7 dB @ 1 MHz and 25.3 dB @ 2 MHz - awesome!
The only thing that I noticed is a distortion of the wave form when using 20 V as supply voltage, that also occurs in the LTspice simulation, even worse there.
Now that seems to come from the high gain trying to amplify the signal beyond the actual supply-voltage. I turned up the voltage to 25 V which almost eliminated the distortion - 30 V were even better! However, that's where R2 whith it's 1/4 W power rating seems to say good by since it heats up quite quickly. I calculated the mean power dissipation with spice and yupp, 350 mW it is. So I guess I will switch to a 1 W resistor on my final amplifier pcb to be on the save side.