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I have the values for this circuit, I guess I didn't think it would matter in determining the kind of load...
L202 22uH
L203 8.2uH
C200 82pF
C201 10uF
R200 560-ohms
Q200 2N3906
I have the values for this circuit, I guess I didn't think it would matter in determining the kind of load...
L202 22uH
L203 8.2uH
C200 82pF
C201 10uF
R200 560-ohms
Q200 2N3906
All the circuits shown above are follow-on stages from the device creating the composite video, which is originally a TM9918A VDP, and now my FPGA-based replacement. I'm just trying to generate composite video that will be compatible with the existing circuitry in all these various computers. The original 9918A has an internal MOS transistor driver with an open source, and expects a 490-ohm external resistor to develop the signal. I could not get a MOS transistor circuit to work for me, so I turned to an opamp since the loads are different in every computer the chip is used in.
Throw normal 75-ohm loads out the window, my circuit is not the end driver going to a monitor. I simply need to develop a linear 0V to 1.92V signal across a variable load, which seems to range from about 390 to 560 ohms *resistive*, but I don't know what the effect the inductors and capacitors will have on the opamp (which is why I posted.)
The first circuit seems to be closest to what the original 9918A datasheet specifies, and looks to be totally resistive, and should not be a problem for the opamp. The other two circuits add the extra inductors and capacitors which complicated the load determination for me.
A basic unity-gain opamp circuit seemed to the be thing I needed, but I made the mistake of reading the datasheet and all the warnings about capacitive loads.
Some questions. Did you try out the recommended reference design given in the datasheet?
You said you couldn't get the MOS transistor cct to work, which one do you mean? The recommended circuit or one of your posted images?
If I didn't miss something, you didn't yet tell about the used OP type nor about the exact problems observed with it.
You mentioned you have FPGA and VDP etc. Are you building a configurable digital or software-defined video switch and repeater in one box or something with digitally-tunable filter?
My suggestion is for you to use a trimmer for R200 (so you may need to tune it, until you got the right value, then change it to a fixed resistor later)
In video, most of the time, you need some manual adjustment to get it right and it is good to have option to allow that.
I see. FPGA is used to behave like 9918A.
OK I think I know where is the problem. Let's put aside the op-amp.
Did you try to use an external MOS transistor (let it driven by your FPGA) and to make it work in Open Source like the internal MOS transistor in 9918A?
I may be simpler this way as it mimics closer to 9918A.
Hmm.... I see. You have to tackle different loads (due to different computer to interface with your hardware design)
Why don't you tackle one interfacing computer at a time? Make one work first. Then move on and modify slightly for another computer?
If you have a negative supply voltage available, a npn/pnp emitter follower combination can also achieve a low voltage shift and moderate output impedance and linearity. But it can't compete with an OP buffer.
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