Forums

Revisit amps input impedance

Started by amdx November 25, 2011
  I got this high input impedance amp out again and redid my input 
capacitor. I have it at about 0.3pf.
http://www.crystal-radio.eu/enfetamp.htm
  The writer says the input impedance is near 5 gigaohms but,
I put in a 1 Mhz 4Vpp signal and got out a 4Vpp signal.
Then I put a ten Meg in series with the input and my output
dropped to about 0.5Vpp. That certainly doesn't seem like
it is anywhere close to 5 Gigaohm.
   So can someone calculate the input impedance, I want to figure out if 
I have something wrong or if the writer got it wrong.

                             Thanks, Mikek

On Fri, 25 Nov 2011 19:31:52 -0600, amdx <amdx@knologynotthis.net>
wrote:

> I got this high input impedance amp out again and redid my input >capacitor. I have it at about 0.3pf. >http://www.crystal-radio.eu/enfetamp.htm > The writer says the input impedance is near 5 gigaohms but, >I put in a 1 Mhz 4Vpp signal and got out a 4Vpp signal. >Then I put a ten Meg in series with the input and my output >dropped to about 0.5Vpp. That certainly doesn't seem like >it is anywhere close to 5 Gigaohm. > So can someone calculate the input impedance, I want to figure out if >I have something wrong or if the writer got it wrong. > > Thanks, Mikek
Are you serious ?:-) ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
"amdx"
> > I got this high input impedance amp out again and redid my input > capacitor. I have it at about 0.3pf. > http://www.crystal-radio.eu/enfetamp.htm > The writer says the input impedance is near 5 gigaohms
** He does not say that at all.
> I put in a 1 Mhz 4Vpp signal and got out a 4Vpp signal. > Then I put a ten Meg in series with the input and my output > dropped to about 0.5Vpp. That certainly doesn't seem like > it is anywhere close to 5 Gigaohm.
** The writes says there is 1.4pF of capacitance at the input. THAT alone has an impedance of just over 100 kohms at 1MHz. Radio frequencies and impedances in the megohms do not go together. FFS learn some math. ... Phil
On 11/25/2011 7:59 PM, Phil Allison wrote:
> "amdx" >> >> I got this high input impedance amp out again and redid my input >> capacitor. I have it at about 0.3pf. >> http://www.crystal-radio.eu/enfetamp.htm >> The writer says the input impedance is near 5 gigaohms > > ** He does not say that at all. > >> I put in a 1 Mhz 4Vpp signal and got out a 4Vpp signal. >> Then I put a ten Meg in series with the input and my output >> dropped to about 0.5Vpp. That certainly doesn't seem like >> it is anywhere close to 5 Gigaohm. > > > ** The writes says there is 1.4pF of capacitance at the input.
Ok, I missed that, he's figuring additional capacitance from his input wires and input connection. Just the 1.4pf has an impedance of about 115K ohms. Then the 0.3pf in series with 20 Megaohm has an impedance not much more than 20 Megaohms, so we have 115k parallel 20 meg, so we are still about 115K. Then throw in the gate impedance and we are slightly less than 115k. So where did the writer come up with: "But the input resistance of the amplifier is much higher then 20 M.Ohm, in theorie even 17&#2013266098; times higher (so, 5780 M.Ohm), this is because over the 20 M.Ohm resistor is only 1/17th part of the input voltage. In practice the input resistance will be lower then 5780 M.Ohm because of dielectric losses e.g. in the gate of the FET." 145k is certainly lower than 5780 Megaohms, but I would not call it high impedance. ??? I'm not sure how the 1.4 pf should be treated, the circuit is used to measure Q of LC circuits, so the 1.4pf could just lower the resonant frequency slightly and the Q would be affected only by the quality of the 1.4pf of capacitance. So maybe this isn't as bad as the 115 kohm impedance suggests, but I'm still stuck with my 10 megaohm experiment that started my question.
> THAT alone has an impedance of just over 100 kohms at 1MHz. > > Radio frequencies and impedances in the megohms do not go together. > > FFS learn some math.
Yes, that would be very helpful, maybe when I retire. If I was starting over I would have studied math and physics. Thanks, Mikek
> > > ... Phil > >
"amdx"

> So where did the writer come up with: > > "But the input resistance of the amplifier is much higher then 20 M.Ohm, > in theorie even 17&#2013266098; times higher (so, 5780 M.Ohm),
** Ask him - you fucking tenth wit. Looks like pure goobledegook to me.
>>FFS learn some math.
> Yes, that would be very helpful, maybe when I retire.
** So what is your profession now - cab driver maybe ?
> If I was starting over I would have studied math and physics.
** Instead of what ? Pure and Applied Bullshit ? ... Phil
"Phil Allison" <phil_a@tpg.com.au> wrote in message 
news:9jcan3F5vqU1@mid.individual.net...
> > "amdx" > >> So where did the writer come up with: >> >> "But the input resistance of the amplifier is much higher then 20 M.Ohm, >> in theorie even 17&#2013266098; times higher (so, 5780 M.Ohm), > > ** Ask him - you fucking tenth wit. > > Looks like pure goobledegook to me. > > >>>FFS learn some math. > >> Yes, that would be very helpful, maybe when I retire. > > > ** So what is your profession now - cab driver maybe ? > > >> If I was starting over I would have studied math and physics. > > > ** Instead of what ? > > Pure and Applied Bullshit ? > > > > ... Phil > > >
LOL classic Phil. I'm printing this one out and sticking it on the pool room wall .
On Sat, 26 Nov 2011 07:26:11 -0600, amdx <amdx@knologynotthis.net>
wrote:

>On 11/25/2011 7:59 PM, Phil Allison wrote: >> "amdx" >>> >>> I got this high input impedance amp out again and redid my input >>> capacitor. I have it at about 0.3pf. >>> http://www.crystal-radio.eu/enfetamp.htm >>> The writer says the input impedance is near 5 gigaohms >> >> ** He does not say that at all. >> >>> I put in a 1 Mhz 4Vpp signal and got out a 4Vpp signal. >>> Then I put a ten Meg in series with the input and my output >>> dropped to about 0.5Vpp. That certainly doesn't seem like >>> it is anywhere close to 5 Gigaohm. >> >> >> ** The writes says there is 1.4pF of capacitance at the input. > >Ok, I missed that, he's figuring additional capacitance from his input >wires and input connection. Just the 1.4pf has an impedance of about >115K ohms. Then the 0.3pf in series with 20 Megaohm has an impedance not >much more than 20 Megaohms, so we have 115k parallel 20 meg, so we are >still about 115K. Then throw in the gate impedance and we are slightly >less than 115k.
It's a silly circuit. A bootstrapped jfet or phemt could have unity gain (no need for all that other junk) and a fraction of a pF of actual terminal capacitance; he has 1.4 pF, mostly parasitic. John
On 11/25/2011 7:31 PM, amdx wrote:
> I got this high input impedance amp out again and redid my input > capacitor. I have it at about 0.3pf. > http://www.crystal-radio.eu/enfetamp.htm > The writer says the input impedance is near 5 gigaohms but, > I put in a 1 Mhz 4Vpp signal and got out a 4Vpp signal. > Then I put a ten Meg in series with the input and my output > dropped to about 0.5Vpp. That certainly doesn't seem like > it is anywhere close to 5 Gigaohm. > So can someone calculate the input impedance, I want to figure out if I > have something wrong or if the writer got it wrong. > > Thanks, Mikek >
The *magnitude* of the input impedance is 94.6k.
On 11/26/2011 8:17 AM, Phil Allison wrote:
> "amdx" > >> So where did the writer come up with: >> >> "But the input resistance of the amplifier is much higher then 20 M.Ohm, >> in theorie even 17&#2013266098; times higher (so, 5780 M.Ohm), > > ** Ask him - you fucking tenth wit. > > Looks like pure goobledegook to me.
Hmm.. his test of connecting a second unit and noting no loading effect on the measurement was evidence of a very high input impedance. But how high is the input impedance? and how to treat the 1.4pf. I'm thinking the 1.4pf is just part of the resonating capacitance and can be ignored as a load (except for the loss resistance)
> >>> FFS learn some math. > >> Yes, that would be very helpful, maybe when I retire. > > > ** So what is your profession now - cab driver maybe ?
Fish Monger. And I'm nice for about it.
> > >> If I was starting over I would have studied math and physics. > > > ** Instead of what ? > > Pure and Applied Bullshit ? > > > > ... Phil
High school Auto body repair. Thanks, Mike
On 11/26/2011 11:43 AM, John Larkin wrote:
> On Sat, 26 Nov 2011 07:26:11 -0600, amdx<amdx@knologynotthis.net> > wrote: > >> On 11/25/2011 7:59 PM, Phil Allison wrote: >>> "amdx" >>>> >>>> I got this high input impedance amp out again and redid my input >>>> capacitor. I have it at about 0.3pf. >>>> http://www.crystal-radio.eu/enfetamp.htm >>>> The writer says the input impedance is near 5 gigaohms >>> >>> ** He does not say that at all. >>> >>>> I put in a 1 Mhz 4Vpp signal and got out a 4Vpp signal. >>>> Then I put a ten Meg in series with the input and my output >>>> dropped to about 0.5Vpp. That certainly doesn't seem like >>>> it is anywhere close to 5 Gigaohm. >>> >>> >>> ** The writes says there is 1.4pF of capacitance at the input. >> >> Ok, I missed that, he's figuring additional capacitance from his input >> wires and input connection. Just the 1.4pf has an impedance of about >> 115K ohms. Then the 0.3pf in series with 20 Megaohm has an impedance not >> much more than 20 Megaohms, so we have 115k parallel 20 meg, so we are >> still about 115K. Then throw in the gate impedance and we are slightly >> less than 115k. > > It's a silly circuit. A bootstrapped jfet or phemt could have unity > gain (no need for all that other junk) and a fraction of a pF of > actual terminal capacitance; he has 1.4 pF, mostly parasitic. > > John
Hi John, How would you treat the 1.4pf? I'm thinking it is just additional resonating capacitance with maybe a little more loss than the capacitor in the LC circuit. You mentioned bootstrap last time I brought this up. I'd be happy to look at a bootstrapped fet circuit if you can put one together. I have LTspice if you want to post a netlist. I have BF256C fets in my little stock. Here's a photo of the layout I have, except I have changed the input 0.3pf cap to a very small piece of Duroid 5880 hung between the fet gate and the input with #32 wire. I also removed the white piece of polypropylene. http://i395.photobucket.com/albums/pp37/Qmavam/Kleijerampinbox.jpg Thanks, Mikek