What are the benefits of this cascode?

He was probably worried about Miller capacitance detuning a high-Q resonant antenna.

Cheers

Phil Hobbs

I hope that "BNC" is a short cable. Or no cable. It's shorted at one end (emitter input impedance ~0) and open at the other (drain output impedance ~infty). It's a 1/4 wave tuned emitter oscillator..

The 2.2k going to the crystal would seem redundant, but I doubt the circuit overall is very optimal anyway. Also, maybe some AGC would be nice, I guess?

Tim

-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website:

In the text he claims the cable should be several meters long in order to move the loopstick from the EMI sources. A good advice, anyway, but not with this type of a circuit.

How would you couple the antenna preamp with the receiver? Just an impedance matching source follower and the amplification at the other end of the cable or some gain close to the loopstick?

He seems to use the limiting amplifier approach to produce the reference frequency for his PLL, so the gain sould be as infinite as possible. ;-)

Best regards, Piotr

Probably just for the large gain. One of the "flaws" with the single active device common emitter amp is that the value of load resistance directly affects the quiescent current and hence the small-signal transconductance so there's a (not that big) theoretical maximum undistorted small-signal gain you can get out of a single stage for a given quiescent current.

With the cascode you have more freedom in setting those parameters and hence can pull more gain out of a single stage without adding the extra noise from two stages

On Aug 26, 2017, Piotr Wyderski wrote (in article ):

. The 77 KHz signal is carried as a current-mode signal, with essentially constant voltage on the coax, so cable capacitance has little effect. I designed a circuit like this for carrying electric guitar signals from the guitar to the guitar amp over up to a thoused feet of generic coax. Works very well. Only complaint was that it sounded like a ?radio mic?, by which they meant that it was too hi-fi.

Joe Gwinn

Why can't the preamp go with the antenna? That's usually how they connect antenna preamps anyway.

It can and should, but the gain can't be set too high. High Q (~100) ferrite antennas can too easy be turned into oscillators.

Best regards, Piotr

I don't know that high gain is required. Usually the issue is driving the line to the receiver with enough gain so the front end of the receiver is not the limiting noise factor. But every application is different.