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How to improve AD834 based VGA circuit

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dzseki:
The ratio of R31/R27 and R32/R28 trims the gain of this circuit (references to my schematic). R31 and R32 also there to decouple somewhat the open collectors of the AD834 preventing unwanted oscillations, so they are better to be not very low values. I had the idea to poke around there...
I increased R31-R32 to 100R, and put an RC network accross to each resistor, I choose 39pF and 33R after some calculations and from the available parts around. The intention was to tune the circuit to around 120MHz. Obviously refinement of the RC values are needed because now I have +1.2dB at 120MHz, but the things have changed in the right direction at least, perhaps adding a coil (making the compensation to RLC) would be benefical to tame high freq peaks (or increasing the opamps feedback even further).

duak:
Interesting - I believe increasing the values of R31 & R32 will reduce the voltage gain of Q4 & Q5, and probably increase bandwidth and stability.  The application note discusses this in the first two paragraphs of p.6-46 but does not go into the model of the circuit showing the parasitic inductances and how to mathematically determine the values.

This circuit is also known as a Folded Cascode and is often used in fast opamps.

I would try using the faster transistors now and adjust R31 & R32 to stabilize the circuit.  I believe the AD834 uses a 1 GHz or faster process so it doesn't make sense to use 200-300 MHz parts for Q4 & Q5 if faster ones are available.  I remember trying to build a low power FM transmitter with 2N2907 or 2N3906 transistors at 100 MHz.  No way - not enough gain.  When I used a VHF transistor from an old radio, it worked perfectly.

dzseki:
Here is a VNA screen capture with the revised tuning network. Now I've used 49R9+15pF across 100R at the emitter of the transistors.
I am somewhat concerned about the sharp peaks in the response above 100MHz, but that might be a VNA  thing (an Agilent E8358A) because it changes suddenly above that point.
Also I may try to further increase the feedback resistor of the LMH6703 to dampen the peak around 300MHz a little, but overall this is already much better than it was.

dzseki:

--- Quote from: duak on May 29, 2019, 05:26:00 pm ---I would try using the faster transistors now and adjust R31 & R32 to stabilize the circuit.  I believe the AD834 uses a 1 GHz or faster process so it doesn't make sense to use 200-300 MHz parts for Q4 & Q5 if faster ones are available.  I remember trying to build a low power FM transmitter with 2N2907 or 2N3906 transistors at 100 MHz.  No way - not enough gain.  When I used a VHF transistor from an old radio, it worked perfectly.

--- End quote ---

I was very thorough checking 2N3906 and MMBT3906 datasheets, and I only found two which contained an ft curve at all, those curves were satisfactory however. One Taiwanian manufactuer (Comchip) suggested that typical ft should be around 500MHz @ Ic=10mA, other datasheet from ROHM suggests also >400MHz around 10mA, sot this should be OK I think.

David Hess:

--- Quote from: dzseki on May 30, 2019, 11:18:36 am ---
--- Quote from: duak on May 29, 2019, 05:26:00 pm ---I would try using the faster transistors now and adjust R31 & R32 to stabilize the circuit.  I believe the AD834 uses a 1 GHz or faster process so it doesn't make sense to use 200-300 MHz parts for Q4 & Q5 if faster ones are available.  I remember trying to build a low power FM transmitter with 2N2907 or 2N3906 transistors at 100 MHz.  No way - not enough gain.  When I used a VHF transistor from an old radio, it worked perfectly.
--- End quote ---

I was very thorough checking 2N3906 and MMBT3906 datasheets, and I only found two which contained an ft curve at all, those curves were satisfactory however. One Taiwanian manufactuer (Comchip) suggested that typical ft should be around 500MHz @ Ic=10mA, other datasheet from ROHM suggests also >400MHz around 10mA, sot this should be OK I think.
--- End quote ---

2N3906s can be selected for low collector-base time constant yielding high Ft (1) however in general, a faster part like an MPSH82 would be used for 100 MHz operation.

(1) Tektronix did this to find 500 MHz parts for 100 MHz designs.

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