dual transistor gate

[cnm]

what are the advantages of using dual gate FET for amplifiers?

any comments/insights are much appreciated.

 

[FvM]

Besides special uses, you can imagine a dual-gate FET as a cascode amplifier (mostly CS-CG) in one component.

 

[cnm]

Cool! What are the differences between using two discrete ones vs.one dual gate?

 

[RCinFLA]

Better isolation, typical use of second gate is AGC.

Advantage is lower cost and part count, maintaining better isolation over two device cascode.

 

[cnm]

Why is the isolation better than two discrete ones?
Thank you for the comments!

 

[vfone]

I don't think that a dual-gate device have better isolation than a dual-FET cascode configuration (at least frequencies below 500MHz), but for sure have less parasitics. In a cascode configuration the connection between transistors is very sensitive to parasitics which can change dramatically the performance of the cascode vs frequency.
So, a dual-gate transistor definitely can reach much higher working frequencies than a cascode. There are some dual-gate GaAs MESFETs can work at microwave frequencies.

 

[cnm]

Sounds like cascade amp is prone to oscillate with parasitics.
Any comments from the device level on why dual gate is better?

 

[goldsmith]

Hi Cnm
In addition , a dual gate mosfet or fet , can be used as an IF amplifier or a mixer stage . or in multiplier stages or divider stages .
Best Wishes
Goldsmith

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There are some dual-gate GaAs MESFETs can work at microwave frequencies.

Hi Vfone
Can you introduce me some of them , please ?
Thank you very much
Goldsmith

 

[cnm]

Thank you, Goldsmith. How does it work when one use dual gate as multiplier or divider?

Dual gate devices were used widely at microwave frequencies (for amp and mixers), although I haven't found any free articles online yet. Anyone who has those care to post a few?

I am still trying to understand how dual gate is better from the device level point of view? There should be changes in the channel properties due to the insertion of two gates, which yield all the benefits.

 

[goldsmith]

How does it work when one use dual gate as multiplier or divider?

Hi Cnm again !
COnsider you use a tuned circuit in drain of a dual gate mosfet and use a low value resistor in source up to ground . and then give a 1MHZ and 2 MHZ to the gates . if both are sine wave , you will have such this series :
1/2 [ cos(w1-w2)-cos(w1+w2)] ====> 1/2[cos(F1-F2)2pit-cos(F1+F2)2pit ] so , you tuned circuit can be selected for getting subtracted or summed signals , thus you will have 3 MHZ or 1 MHZ . so you can select 3 MHZ . you will have a 3 MHZ in out put . a notice : if you use that device in nonlinear region , you can do every thing that you want with that ! ( such as giving two distorted signals and then taking other harmonics .
Good luck
Goldsmith

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In addition , if you give 200 MHZ and 300 MHZ , to the gates , you can take 500 MHZ or 100 MHZ !

 

[cnm]

Thank you Goldsmith --mixer then (or multiplier in time domain).