Greetings ### I'm slowly working on circuits for my next video. I sought a common gate amp with a well-defined, wide-band, output impedance to go on the RF port of a diode ring mixer. I also matched the input to 50 Ω for the entire 40M amateur radio band.
I'll show more hand drawn schematics going forward. I only post a fraction of my experiments -- partly, because I'm lazy; but it takes considerable time to draw schematics with software. Thus, I often don't bother sharing much content nor post that much. My writing is Ugly style.
Above — Amplifier measures. Gain = 6.43 dB. From my experience, you'll only get the often mentioned ~10 dB gain when you tune the drain -- plus run an air wound, or a dust-powder toroid like the T50-6 on the output tuned circuit. The #43 ferrite exacts losses, but works OK for wide band transformers -- and in combination with the 2K0 resistor -plus- a drain ferrite bead, or 22-51 Ω drain resistor, behaves itself. (Does not usually oscillate).
Typically, we keep the FET's gate lead length to the ground plane short [ low inductance] to boost stability as the J310 is a VHF-UHF part.
The common gate or common base topology output impedance proves difficult to well match to 50 Ω using just transformer turns ratios. The usual method is to place a resistor across the coil in parallel with the primary coil. This works well & I chose 2K0 Ω for that resistor.
The turns ratio reflects the Z transform from 2K to 50 Ω. I measured the output return loss from 2 to 13 MHz . The output return loss stayed above 20 dB from 2.1 to 12.8 MHz.
Then I sweep tested from 7.0 to 7.4 MHz. The output port return loss measured 22.8 dB across the whole 40M band. I'm happy. A decent match for my product detector RF port.
To measure return loss on the input port, we place a 50 Ω terminator on the output port. Then to measure the output port return loss, we switch ports-- and place the terminator on the input port.
Above — A wide band BNC 50 Ω terminator. I've got these and SMA versions that go from DC to 18 GHz. You may also make your own with a BNC/SMA connector and resistor(s). We use them to measure the directivity of our return loss bridges -- and calibrate stuff like VNAs.
Note in the data, I performed 1 more measure on the input. I checked the input return loss with no 50 Ω terminator on the output port - open circuit. The return loss across the 40M band = 18.6 dB with an open circuit output. Amazing. Recall, that many RF amplifiers are termination sensitive.
Termination sensitive means that the impedance or resistance seen at the output port may affect the input impedance. In this case, the amplifier input return loss remained ~OK despite the output port going open circuit.
Further, I measured the reverse isolation of this amp at 32.5 dB. I believe strong reverse isolation is the primary reason that the amp seemed fairly termination insensitive.
This amp seems special -- and it is. The common base (BJT), common gate (FET), and a cascode of 2 FETS, BJTS or a FET + a BJT offer better reverse isolation than standard common emitter/ common source amps.
Above — A sweep in my tracking generator-spectrum analyzer. This device says the gain = 6.65 dB @7 MHz . The low Q pi network offers some low pass filtration - bonus points.
This RF preamp will go in a HF direct conversion receiver. In the past 25 + years when I've done this -- on 3 occasions, total strangers took the time to email me & wrote that I'm trashing my receiver dynamic range -- that I don't need it -- and to get it out of there!
Oh no -- its the RF preamp police lol!
I'll say it here -- it's OK to put in a RF preamp in an HF receiver if you want to. Attenuate or boost the signal before the product detector/mixer if you like. You do you. There may be a good case for RF preamps at HF in some circumstances.
Throughout solid state amateur radio history, some rather clever builders placed common gate (or common base) preamps in front of direct conversion or regenerative receivers to reduce the amount of LO signal at the RF port going out into the antenna, doing bad stuff and ending up back in the mixer. (I'll be covering this in the video).
Perhaps, the listener receives with a subpar antenna? For example, with a small loop antenna -- and needs a little bit of RF gain? Further, not everyone is an amateur radio contest listener. Many just like to tinker with simple circuits and enjoy making signals bigger.
Above — Another case? The transmit output amp of my old 7 MHz Funster trans-receiver. The receiver is connected to the antenna via a low Q series-tuned network. During transmit and a for a few milliseconds after, the pin diode pair shunts the PA RF output to ground.
On receive, the signal from the antenna passes through the low pass filter, then through a XL = XC = ~400 Ω series tuned network ( 56 pF + 9.1 µH ) to the receiver input. The series resonant circuit + low pass filter exhibits 8.8 dB of losses. A 6-8 dB gain common gate amp might help builders who choose the series connected transmit/receive scheme shown above.
An RF preamp is probably not required in most cases for HF reception. However, there are always exceptions. A strategically placed amplifier with high reverse isolation might prove helpful and boost your receiver function and enjoyment. Ultimately, you're in charge. Do whatever you like.
Best to you!
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