Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives
Support This Site: Contributors :: Advertise


It is currently Sep Mon 16, 2019 11:18 pm


All times are UTC [ DST ]





Post New Topic Post Reply  [ 7 posts ] 
Author Message
 Post subject: I have a question regarding gain using JFETS
PostPosted: Mar Mon 19, 2018 8:10 am 
Member

Joined: Aug Sat 13, 2016 6:03 am
Posts: 181
Location: Ashland Oregon
All things being equal which configuration produces the largest gain for a small signal input. A common gate or common source?

I have seen all the equations which would have totally confused me years ago and are now out of the question. I expect that some of you have discovered this answer. Hoping for a simple answer. Thank you michael


Top
 Profile  
 
 Post subject: Re: I have a question regarding gain using JFETS
PostPosted: Mar Mon 19, 2018 2:19 pm 
Member
User avatar

Joined: Jul Mon 26, 2010 8:30 pm
Posts: 26382
Location: Annapolis, MD
Do it on first principles. The small-signal drain current is the transconductance (Gm) times the gate to source voltage. So the common-source gain is Gm X R, where R is the net effective resistance from drain to source.

With either common-source or common-gate, the gain is modified by any resistance that is shared between the input and output circuits. IOW, any resistance in the path of the drain current that also affects the gate-source voltage. This is effectively a negative feedback within the circuit.

So, for either common-drain or common-gate, the gain is less than for common-source. There is a chart of equations in a recent thread about the output impedance of a JFET circuit.

_________________
-Mark
"Measure voltage, but THINK current." --anon.


Top
 Profile  
 
 Post subject: Re: I have a question regarding gain using JFETS
PostPosted: Mar Mon 19, 2018 2:21 pm 
Member
User avatar

Joined: Jul Mon 26, 2010 8:30 pm
Posts: 26382
Location: Annapolis, MD
PS
All things cannot be equal....each of the three configurations uses a different circuit and resistance values.

_________________
-Mark
"Measure voltage, but THINK current." --anon.


Top
 Profile  
 
 Post subject: Re: I have a question regarding gain using JFETS
PostPosted: Mar Mon 19, 2018 2:24 pm 
Member
User avatar

Joined: Jul Mon 26, 2010 8:30 pm
Posts: 26382
Location: Annapolis, MD
It was your thread where the table was posted!!

viewtopic.php?f=12&t=336219&p=2828443#p2828443

Look at the post from findem keepm

_________________
-Mark
"Measure voltage, but THINK current." --anon.


Top
 Profile  
 
 Post subject: Re: I have a question regarding gain using JFETS
PostPosted: Mar Mon 19, 2018 6:21 pm 
Member

Joined: Aug Thu 06, 2015 2:20 am
Posts: 175
In simplest terms, the Common Source configuration has more Voltage gain.

As can be seen by the equations in your other thread, the gain of a Common Gate amplifier contains terms in the denominator that are dependent on the impedance of the input signal.

The advantage to the Common Gate configuration is that its input and output are isolated from each other which is an advantage at high frequencies because of minimal Miller capacitance effects and lower input impedance.

The lower gain of the Common Gate amplifier can be made similar to the Common Source configuration by adding an extra FET in cascode which retains the high frequency benefits of the Common Gate configuration with gain similar to Common Source.


Top
 Profile  
 
 Post subject: Re: I have a question regarding gain using JFETS
PostPosted: Mar Tue 20, 2018 5:11 am 
Member

Joined: Aug Sat 13, 2016 6:03 am
Posts: 181
Location: Ashland Oregon
Thank you LM386 for the answer I was hoping for. And it was nice of you to explain why in simple terms I can understand. Yes Mark, I did see all those equations on the chart in my other thread and I figured the ones with more symbols were probably going to produce a lesser voltage gain but most all of this is still magic to me. Alfredo_T I found the statement your engineer/professor friend told you to be quite accurate, at least in my application. Thank you for sharing this.

I'm happy to report I have successfully constructed a balanced common gate RF amplifier. All of your replies were invaluable when it came to winding the toroid transformer. Fine tuning the secondary was by trial and turned out to be very definitive.

Although the overall gain is more than adequate, I thought a common source design might prove to be a little more. I'll start on one in the morning and we'll see. I wouldn't be having this much fun without all your help. michael


Top
 Profile  
 
 Post subject: I have a question regarding gain using JFETS
PostPosted: Apr Fri 13, 2018 2:03 am 
New Member

Joined: Apr Thu 05, 2018 12:46 pm
Posts: 4
Location: Россия
Look i dont know what Nebulosity is up to, but you can set the gain and offset in the camera ASCOM driver.


Top
 Profile  
 
Post New Topic Post Reply  [ 7 posts ] 

All times are UTC [ DST ]


Who is online

Users browsing this forum: No registered users and 2 guests



Search for:
Jump to:  




























Privacy Policy :: Powered by phpBB