@ericgibbs

Hello again Eric
Great job thank you very much !

It raises some questions and unfortunately the webmaster informed me that until I reach 10 post I can't send a private message.
Could we contatct on Linkedin?

Hello Eric,

Here are my questions :

1 -I think you are using a software like LT spice, which one is it ?

2 - Could you explain me what are the roles of C3, C4 and C5 ?

3- On the left of R3, there is an arrow. Where should it be connected to ?

4- The 2 circuits are connected together or is it on the left just an example to get a VG (Virtual Ground ?) ?

Thank you !

Philippe

 

hi P,
1 -I think you are using a software like LT spice, which one is it ?
LTSpice XVII

2 - Could you explain me what are the roles of C3, C4 and C5 ?
Smoothing and Filter caps, for the DC voltages

3- On the left of R3, there is an arrow. Where should it be connected to ?
You see R3 is enclosed in a dotted line rectangle, for the Simulation ONLY, not used on the actual circuit.

4- The 2 circuits are connected together or is it on the left just an example to get a VG (Virtual Ground ?) ?
The left circuit is the method of creating a dual supply of +/-4.5Vdc from a 9V battery.
Note: the +4.5V and -4.5V are used to supply the right hand OPA

E

 

hi P,
1 -I think you are using a software like LT spice, which one is it ?
LTSpice XVII

2 - Could you explain me what are the roles of C3, C4 and C5 ?
Smoothing and Filter caps, for the DC voltages

3- On the left of R3, there is an arrow. Where should it be connected to ?
You see R3 is enclosed in a dotted line rectangle, for the Simulation ONLY, not used on the actual circuit.

4- The 2 circuits are connected together or is it on the left just an example to get a VG (Virtual Ground ?) ?
The left circuit is the method of creating a dual supply of +/-4.5Vdc from a 9V battery.
Note: the +4.5V and -4.5V are used to supply the right hand OPA

E

Eric,

Thank you again you are so fast and nice to answer

Last list of questions !

5- The 2nd op amp is a non-inverting amplifier, right ?
6- How to you calculate the gain ? 1+ R2/R1 ? = 441
7- What if I reduce R1 to 2,2 k to get x 1 000 gain ?
8- What does Mtr 1000 means ?
9- What are the advantages of replacing LM358 with CA 3140 ? For example this IC has 2 pins for an offset voltage nulling and you don’t use them ?
10 -And the most important : how to connect the existing left part of my circuit and the 100 ua galva to your circuit ?

 

hi.

5- The 2nd op amp is a non-inverting amplifier, right ?
Yes.
6- How to you calculate the gain ? 1+ R2/R1 ? = 441
Correct.
7- What if I reduce R1 to 2,2 k to get x 1 000 gain ?
I would recommend that you try increasing the gain to 1000, after you have it working with present gain level.

8- What does Mtr 1000 means ?
That is a LTSpice load resistor that represents your meter, I have assumed a resistance of 1000 ohms.

9- What are the advantages of replacing LM358 with CA 3140 ? For example this IC has 2 pins for an offset voltage nulling and you don’t use them ?
The CA3140 is a CMOS high input impedance Non Inverting OPA

10 -And the most important : how to connect the existing left part of my circuit and the 100 ua galva to your circuit ?

Connect the meter in place of the Mtr 1000 ohm resistor, bear in mind what Dick said about protecting the meter movement.

Your present circuit requires a modification, the Non Inverting must have a resistive path to the 0v common, say 1mega ohm

E

 

The input bias current of a LM358 opamp is 0.25uA max. Then into a 1M to ground input resistor the input voltage without a signal is +0.25V.
The CA3140 Has a Mosfet input with NO input bias current. Its input leakage current is only 50pA (50 micro micro amps).

 

A gain of 1000 is a lot to ask of a single stage opamp. You lose gain as frequency gets higher. Do it in stages.

 

A gain of 1000 is a lot to ask of a single stage opamp. You lose gain as frequency gets higher. Do it in stages.

The frequency does not matter since the detector output is a DC voltage.

 

The frequency does not matter since the detector output is a DC voltage.

Sorry, I did not look at the application.

 

Sorry, I did not look at the application.

Another member in post #16 thought the opamp was used to measure the very low level of 144MHz!

 

Hi Philippe,
This is the detector part of the circuit.
The V1 signal is a test Sine wave at 144Mhz at 10uVppk level, as test signal ONLY.

Note, I do not have your Diode type, so I have used an ideal diode.

E

 

Hi Philippe,
This is the detector part of the circuit.
The V1 signal is a test Sine wave at 144Mhz at 10uVppk level, as test signal ONLY.

Note, I do not have your Diode type, so I have used an ideal diode.

E

Hi Eric !

Thank you so much !

That is exactly the schematic I was going to send to you !

The 1 M resistor in parallel with the 470 pF is an excellent idea for both reasons :
- The 100 nF I used before had a too high value. In fact if there is no VHF field, due to the high impedance of the CA3140, the capacitor would have no time to discharge !
- The 1 M resistor allows the capacitor to be discharged

That is really a great circuit now !

I will have to do now my homework and once the new circuit is ready I'll send you a photo with my comments.

Thank you again for the rich and instructive discussions.

I would like to thank too @DickCappels and @Audioguru and @Irving again for their contributions !

 

Hi Philippe,
This is the detector part of the circuit.
The V1 signal is a test Sine wave at 144Mhz at 10uVppk level, as test signal ONLY.

Note, I do not have your Diode type, so I have used an ideal diode.

E

Hello Eric

Hope you have had a nice week-end.

I have to order some CA 3140. I have 2 CA3130 in stock only.
What is the best one to order 3130 or 3140 ?

Thank you
Philippe

I

 

hi P,
Either type should be suitable, both have high impedance inputs, approx 1Terra ohm.
E

 

hi P,
Either type should be suitable, both have high impedance inputs, approx 1Terra ohm.
E

OK thank you !

Some components are starting to be in shortage ...

Good evening