# Realizing Inductor with a Gyrator circuit

Thread Starter

#### shivashankar_p

Joined Nov 10, 2015
50
I tried to construct a Gyrator circuit (Figure shown below) to obtain a floating inductor. I used OPA445 to implement the circuit. After completing the circuit on the breadboard, I put a resistor (1KOhm) in series with the Inductor and measured the AC voltage across the resistor (to validate the Inductor). The voltage measured across the resistor didn't match the theoretical predictions, and the output pattern itself was erratic. By erratic I mean that there was no consistency in the error; With changing input frequency, sometimes the signal looked distorted, had a DC bias, and the amplitude didn't change.

I feel like I am missing something. Can someone point out what's wrong with this implementation?

Thanks,
Shiva

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#### crutschow

Joined Mar 14, 2008
34,035
Can someone point out what's wrong with this implementation?
What are the resistor and capacitor values?

Thread Starter

#### shivashankar_p

Joined Nov 10, 2015
50
Thread Starter

#### shivashankar_p

Joined Nov 10, 2015
50
1Kohms and 47nF.
Also, Is OPA445 ideal for this circuit? Or should I use some other OpAmp?

#### michael8

Joined Jan 11, 2015
407
The picture has a pin of an IC connecting to the capacitor and nothing else, how
does this match the schematic?

Do the opamp's need bypass capacitors on the power pins?

Are the 1K resistors in the picture too low in value for the opamp to drive?

Whats the power supply? Voltage?

I don't see "rtune", perhaps it's out of the picture. I'd beware of long leads in the gyrator paths
both on the circut board (lots of parasitic capacitance between parts) and off board leads.

Oh, looking at the OPA445 data sheet: https://www.ti.com/lit/gpn/opa445

pin 3 is +in and it's the pin which has only a capacitor connected -- this can't be correct.

I don't know the source of the schematic, however I notice that there is no DC block
at the input so any DC input will alter the opamp bias point(s).

Last edited:

#### AnalogKid

Joined Aug 1, 2013
10,943
The circuit on the proto-board is not the circuit in the schematic.

Also, please post a link to the source for the schematic.

ak

Thread Starter

#### shivashankar_p

Joined Nov 10, 2015
50
The picture has a pin of an IC connecting to the capacitor and nothing else, how
does this match the schematic?

Do the opamp's need bypass capacitors on the power pins?

Are the 1K resistors in the picture too low in value for the opamp to drive?

Whats the power supply? Voltage?

I don't see "rtune", perhaps it's out of the picture. I'd beware of long leads in the gyrator paths
both on the circut board (lots of parasitic capacitance between parts) and off board leads.

Oh, looking at the OPA445 data sheet: https://www.ti.com/lit/gpn/opa445

pin 3 is +in and it's the pin which has only a capacitor connected -- this can't be correct.

I don't know the source of the schematic, however I notice that there is no DC block
at the imput so any DC input will alter the opamp bias point(s).
Thanks for the reply. Please find the answers for your questions below.

1) The red wire connects the capacitor to the output of the third Op-Amp.
2) regarding the 1K resistors, I am not sure whether they are too low for driving. I will replace them and check again.
3) Power supply was 35V between the +V and -V of the Op-Amps
4) rtune is also 1Kohm. It is connected between the -In and OP of the first OPAmp
5) pin 3 is +in and it's the pin which has only a capacitor connected -- this can't be correct. Can you explain why? I did the simulation, and this circuit generates an Inductor.
6) Regarding the long leads, I will try to implement this on a PCB.
7) Source for this circuit: 10.1109/TCS.1976.1084186

Shiva

Thread Starter

#### shivashankar_p

Joined Nov 10, 2015
50
The circuit on the proto-board is not the circuit in the schematic.

Also, please post a link to the source for the schematic.

ak
Thanks for the reply.

Please find the source for the circuit in this link 10.1109/TCS.1976.1084186 .
The circuit in the board corresponds to the schematic. I used the red wire to connect the capacitor to the third opamp & the orange wire to connect the +In of the fist opamp to the OP of the third, via a resistor.

#### Audioguru again

Joined Oct 21, 2019
6,608
You have a wire missing that connects pin 2 of the 1st opamp to pin 3 of the 2nd opamp.

Why do you think that high frequency opamps can work being far apart with the high stray capacitance between rows of contacts on a Mickey Mouse solderless breadboard? Instead use a compact pcb with power supply decoupling capacitors for each opamp.

Your circuit is too complicated. Look here:

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#### crutschow

Joined Mar 14, 2008
34,035
3) Power supply was 35V between the +V and -V of the Op-Amps
The power supply has no ground reference, which is a big no-no. There is no path for the op amp input and output signal currents to get back to the supply.
Use two 15V supplies with one end of each supply grounded (one plus, one minus).

Thread Starter

#### shivashankar_p

Joined Nov 10, 2015
50
You have a wire missing that connects pin 2 of the 1st opamp to pin 3 of the 2nd opamp.

Why do you think that high frequency opamps can work being far apart with the high stray capacitance between rows of contacts on a Mickey Mouse solderless breadboard? Instead use a compact pcb with power supply decoupling capacitors for each opamp.

Your circuit is too complicated. Look here:
Indeed there is a wire missing. Thanks for pointing that out.

Thread Starter

#### shivashankar_p

Joined Nov 10, 2015
50
Thanks for the reply. Please find the answers for your questions below.

1) The red wire connects the capacitor to the output of the third Op-Amp.
2) regarding the 1K resistors, I am not sure whether they are too low for driving. I will replace them and check again.
3) Power supply was 35V between the +V and -V of the Op-Amps
4) rtune is also 1Kohm. It is connected between the -In and OP of the first OPAmp
5) pin 3 is +in and it's the pin which has only a capacitor connected -- this can't be correct. Can you explain why? I did the simulation, and this circuit generates an Inductor.
6) Regarding the long leads, I will try to implement this on a PCB.
7) Source for this circuit: 10.1109/TCS.1976.1084186

Shiva
I did miss a connection with the capacitor. Thanks for pointing it out