Dual gate FET Mixer Design

[ttkane]

now, i have one componet bf998
how can i define dual gate fet mixer circuit gate 1 and gate 2 in RF and LO terminal input

have any design rule about below? or any difference features
gate 1 in RF / gate 2 in LO
or
gate 1 in LO / gate 2 in RF

 

[goldsmith]

Hey guy !
Hi
I think it is not important, because your aim is mixture between Your income frequency and your local oscillator frequency and get fosc-fi and fosc+fi. but it is not important which gate. because each gate can control channel of your Jfet .
for example if you do this thing with diode , is it important??!!!! no . you can test it. your aim is multiply . ( between your amplitudes) and it is not important that A*b or B*A
Best wishes
Goldsmith

 

[ttkane]

thanks for your best reply
i still have an question
if using depletion mode mosfet to built mixer circuit
do it need mount dc bias resistance to gate or do not?

 

[goldsmith]

Again Hi
Depletion mosfets are not good choice for this aim. because in usual mode , they are turn on . and if you give signal to their gate source junction , they will going to cutt of mode.(in reverse with enhancement mode). thus i think your power dissipation will High. but if you want to use depletion mosfets , i think their gates should connect to ground with a resistor . but i'm not sure. because for n channels of depletions, positive voltage can cutoff channel.
Regards

---------- Post added at 13:58 ---------- Previous post was at 13:48 ----------

Yes you will need biasing resistors Go to this site:**broken link removed**

 

[vfone]

Regarding initial question, usually in a dual-gate MOSFET mixer, on the gate-1 is injected the RF signal (lower level) and on the gate-2 is injected the LO signal (higher signal). This is related to the internal structure of dual-gate MOSFETs.

Meantime, the equivalent of a dual-gate FET mixer is to use two FETs mounted in cascode configuration. In this situation, the RF and LO input levels could be equal.

 

[goldsmith]

Dear Vfone
Hi
It is not difference between g1 rf g2 fi or g2 rf g1 fi in practical.
Because our purpose just is multiplying between amplitudes. sina*sinb=sinb*sina=1/2cos((fa-fb)2pt-(fa+fb)2pt) . frequency of fa is higher than fb. and we have : va*va+vb*vb+2va*vb thus its out put again is an amplitude modulation.
so it is not any difference between g1:rf or g2: rf . why transistors are multiplyer: because they are logarithmic and each logarithmic function is a multiplyer.
Regards
Goldsmith

 

[tsvetelin_velkov]

Hi,

I just want to share those docs here. I think they might be usefull to ttkane and other that want to make design using dual-gate mosfets. The dual-gates are very good in RF applications, almost any TV tuner in the world has a preamp stage made with dual-gate mosfet. In the past they were used as mixers, if-amplifiers, product detectors, so almost in any part of the rx/tx system. Now a days they are not that much used because everything is integral.

Just to add my opinion about G1/G2 usage - G1 is used to fed the low power signal or as RF in, G2 is used for biasing or as LO input. Looking at pspice model of BF998, the two gates are identical, the dual-gate mosfet is basically a cascode. I see no reason why not to invert the usage of the gates. However, the package layout is optimized for this application, so if you swap the gates it might not be very optimal for the PCB layout.

I have some service manuals for old radios, they use fet transistors as first mixer, and dual-gate mosfets as if amplifiers and AGC amplifiers.

Enjoy the attachments :