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I've been attempting to build a VCO recently (around 50Mhz) and I've reached the part where I need a solid buffer for my oscillator. Looking around at various options I can't help noticing that many of the buffer amplifiers used in designs are actually of the CCVS type. Take a look at these:

G3PDMs Vackar circuit with CCVS style feedback buffer enter image description here

Wideband MMIC configuration with collector feedback style CCVS buffer amp enter image description here

Taken from here

RF buffer taken from mid-80's copy of the ARRL handbook featuring a collector feedback style buffer amp with impedance transforming pi-filter enter image description here

What's intriguing about all of these configurations is that they have an intrinsically low input impedance. If you take the 2nd circuit (MMIC) as an example. The circuit aims for a high trans-conductance, that makes the trans-impedance close to 560. This gives a gain of approx 11 with a 50 ohm source. But, it has a very low input impedance and the higher you make the open-loop gain, the lower it gets. Even with a 50 ohm source, the impedance is very low, I think.

So, what am I missing here? Why are these shunt type configurations so attractive in RF circles? Does it have something to do with standing wave reflection?

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  • \$\begingroup\$ Of the three circuits you have shown only two have "usable" inputs and one is high impedance and the other is low impedance so what is your (perhaps re-jigged) question based on my observation? \$\endgroup\$ – Andy aka May 16 '18 at 16:15
  • \$\begingroup\$ Fair enough, the G3PDM circuit does have the 5k6 resistor and I suppose the ARRL circuit has a JFET input shunted by a 5k6 feedback resistor. That said, why go for an arrangement where you fight against a "natural" low impedance input? Why not simply go for a high Z input? \$\endgroup\$ – Buck8pe May 16 '18 at 17:21
  • \$\begingroup\$ The first circuit has a tank parallel to the high input impedance of the bootstrap amplifier but can only drive a load R > Re. The second assumes you have a 50 Ohm source. Each has a different Iq, Vout, Zout, what are your specs? \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 May 16 '18 at 17:25
  • \$\begingroup\$ @Tony In the case of the circuit expecting a 50 ohm source, I calculated an Zin of less than an ohm (I may have that wrong) and in the case of the first circuit it's 5k6, but after that, NFB will reduce the impedance to a small value, will it not. I suppose I assumed that your "buffer" would do better to absorb as little power from the source as possible and therefore present a high impedance. \$\endgroup\$ – Buck8pe May 16 '18 at 17:39
  • \$\begingroup\$ There are plenty of reasons to have an amplifier with virtually zero ohm input impedance. If that is your question then please ask it. \$\endgroup\$ – Andy aka May 16 '18 at 18:15
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In the Vackar, the well-defined Rload may precisely set the stable-point amplitude of oscillation.

In the other 2 circuits, I can see a value in having circuits with high S12, or high reverse isolation, to minimize the loadpull.

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    \$\begingroup\$ Yes, the output amplitude does depend on the load (and quite complex it is too!). However, having built the circuit I can say that the amplitude varies quite a bit as you tune to different frequencies. In the end, this was for a variety of reasons: changing Q, early effect with intrinsic drain resistance, etc. I failed miserably to predict the output - but that as they say, is a different story. \$\endgroup\$ – Buck8pe May 17 '18 at 6:17

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