active antenna LNA

[orcus]

I am designing a active antenna (100KHz). Can I use AD 797 (ultra LNA) as the amp instead of JFET. the input output impedance of this opamp are similar to that of JFET (high input imp and low output imp).
I have not seen any design implementing opamp in active antenna and was curious if there are any problems.

 

[zappedagain]

After a quick look at the AD797 data sheet it looks like a good candidate. With a positive gain configuration (Figure 25) you'll only need to match to the input impedance; the op-amp will take care of bias and such.

The gain is down to 60 dB at 100 KHz; though the phase margin still looks good. How much gain do you need?

Op amps get a lot of use as transimpedance amps. There are some good books out there that you might reference.


On the other hand...

You might find better data for the input impedance of a FET; the op-amp will require experimentation.

 

[BrianR]

The input capacitance might be an issue here. Active antennas are generally very short electrically and their input impedance is capacitive, R-jX where R is VERY low and -jX is VERY large. This means the input capacitance of your amplifier forms a voltage divider and the 20pF of the AD797 seems too large. I have seen active antenna amps using which a FET followed by a bipolar transistor connected in a bootstrap configuration resulting in an input C of fractions of a pF.

 

[VEBill]

I agree with Brian.

I note that the designers of Active Antennas always take great pains to keep the antenna element away from ground. The whip is always mounted directly in a manner (how can I put it?...) diametrically opposed to feeding it through a length of coax.

The input impedance, both R and C, needs to be extreme (high R, low C).

 

[logbook]

Who says the antenna is a monopole? A 100kHz antenna is probably a loop. In which case the opamp is fine and the capacitance is irrelevant.

 

[VEBill]

The term 'Active Antenna' is normally used to refer to a short whip, intended for VLF to HF reception, maintained at very high impedance (not fed with coax) and fed (usually) directly into the gate of some type of a FET.

I've got two around the house, a Sony AN-1 (broadband) and an MJF-1020A (tuned). They work 'hotdamn' on VLF to HF. But they do not offer any directionality as a small loop might.