Darlington replacement in HF preamplifier
In my 1MHz-30MHz receiver, http://qrp.gr/allbander/index.htm I am trying to achieve more amplification with simple devices (not MMIC) out of the RF preamplifier BC109C, without changing the 4.7pF capacitor.
I consider a darlington transistor (BC517) instead of the BC549c. Keep in mind that everything has to be kept simple, it is a hobbyist circuit.
Will I see any gain improvement up to 30MHz compared to the BC549C, or it does not worth the effort?
Darlington transistors are useless for RF applications. Don't expect gain improvement, rather function loss.
Even though BC517 is specified to an Ft of 200MHz?
bc109c ft=150mhz
bc209c ft=200mhz
bc549c ft=250mhz
bc517 ft=200mhz hfe=30000 (min)
You need to consider the meaning of the term fT. It's the frequency where current gain falls to unity. Respectively the RF gain of a darlington is at best similar to a standard transistor. But you most likely won't be able to utilize it at all.
A certain gain improvement for the discussed circuit might be achieved by using a dedicated RF preamplifier transistor, due to reduced reverse transfer capacitance.
Thank you.
I have some BF199. I will try them but do you expect them to achieve more RF amplification than the BC549C?
BF199 has 85 hfe (at dc) and bc549c has up to 700 or so.
I don't know exactly how does it work, but you are using positive feedback to get more gain, aren't you? Because it seems to a regenerative circuit. And maximum positive feedback limits the loop gain before oscillation.
So you should just increase the positive feedback to get more gain until the oscillation, and if it is not possible, you should rather increase the sensitivity by decreasing the noise figure of the preamp.
Choose a transistor which has got lower noise figure maybe, or increase the current consumption of it.
And Darlington transistor won't increase your gain I think, just the input impedance at the base will increase. Darlington's parasitics are higher (junction and diffusion capacitances also). If you don't need high input impedance it is not necessary.
In this configuration I consider only the RF preamp gain to be increased. I am quite happy from the regenerative detector stage.
I see that all the RF transistors have low Hfe, but this is specified for the RF frequency not DC.
Here is a very useful list I have found just now and I was looking for years!
I am confused by the so many params, maybe I will just try and see.