Re: Why JFET amp instead of Non-inv Op amp? Do they both not have high Imp?
- From: John Popelish <jpopelish@xxxxxxxx>
- Date: Tue, 12 Apr 2005 12:03:46 -0400
Stephen wrote:
Thanks for the reply.
I understand that gain will go down with higher frequencies in accordance with the gain bandwidth product. What about the input capacitance issue? Does this increase with increasing frequency? I would be using a non-inverting op-amp so there would not be any miller effect. Where does the extra capacitance come from?
No, the capacitance does not go up with frequency. But RF fets can have lower input capacitance than op amps, in many cases. Common source fet amplifiers are also dogs in many cases, and cascode or common drain configurations or other combinations may be needed to get reasonable gain at RF.
Can you specify what RF frequency range you are talking about?
In general, a chain of amplifier stages (there are at least 3 in most op amps) with overall feedback will not have as high a gain bandwidth product than a series of individual device stages with no overall feedback at the signal frequency because of the accumulated phase shift. This is why op amps have internal compensation capacitors that provide a dominant pole of roll off to get the gain below 1 before the total phase shift reaches 180 degrees. A series of independent stages do not have this phase shift limitation.
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