negative resistance with conveyor current

[azerty1]

Hello everyone,
I'm working on a project on current conveyors and I want to test the operation of the latter in negative resistance. My question is that I have to attack the Y input conveyor with a current generator and a voltage generator? to have a 180 ° phase shift between the voltage and the current input .
Thank you bcp.
You will find attached the diagram of the negative resistance.

 

[LvW]

Are you sure that the circuit behaves like a negative resistance?
Where does this circuit comes from?
No external components ?

 

[azerty1]

hello,
yes, this is what I found featured on a PDF file. as you will find attached in page 11.

http://cas.ee.ic.ac.uk/people/dario/files/E416/cc_handout07.pdf

 

[LvW]

hello,
yes, this is what I found featured on a PDF file. as you will find attached in page 11.
http://cas.ee.ic.ac.uk/people/dario/files/E416/cc_handout07.pdf

Yes - in the mean time I have convinced myself that - indeed - the non-inv. y-input shows a negative input impedance.
In principle, the CCC behaves like a conventional CCII+ (like AD844).

 

[FvM]

I don't understand what's your problem in circuit test. You can either apply a voltage and measure current or vice versa. To gurantee stability, the negative resistance must be parallel connected with a resistor < |R|.

 

[FvM]

As an additional comment, the circuit in post #1 misses a resistor to set the negative resistance value.

 

[LvW]

As an additional comment, the circuit in post #1 misses a resistor to set the negative resistance value.

Hi FvM, I am not sure about this because I am not familiar with all CCC features (external control of transfer properties).
As an example: The artificial resistor realized with an OTA (to be controlled via Iabc) has the value R=1/gm without any external part.
And, for my opinion, the CCC circuit does the same - in principle - however, with an output current that has another polarity as the OTA output (therefore: negative resitor).

 

[FvM]

Yes, without a resistor, you get Zin = -1/gm. Also an OTA can be used to implement 1/gm or -1/gm impedance.

But gm must be sufficient low and real impedance will be only observed below the first gm pole.

 

[azerty1]

I try to make simulations attacking the entrance with a voltage generator and as a current generator and the two cases when I display the voltage and current input I do not see the sign reversal between these two signals.

 

[LvW]

I try to make simulations attacking the entrance with a voltage generator and as a current generator and the two cases when I display the voltage and current input I do not see the sign reversal between these two signals.

Which type of simulation? DC or AC or TRAN?
What kind of active device are you using?
Show us the simulation arrangement.

 

[azerty1]

I do a TRAN simulation to verify that I have a reversal between the input voltage and current, and i do AC simulation to see if I have a 180 ° at phase, which are the characteristics of a negative input resistance .
And I work with the current conveyor based bipolar transistors which is in file that I have already sent.

 

[FvM]

You mean page 6 "Push-pull CCI topology"?
Maybe wrong DC bias in simulation?

 

[LvW]

Azerty1, I have used the AD844 unit (CCII+ type) for simulation purposes.
The (low-resistance) inverting input (x) was grounded and the (high-resistance) output connected to the non-inv. y-input.

For both analyses - tran and ac - a small input voltage (1m) at the y-input did cause a current into the circuit with 180 deg phase shift.
I didn`t check if the BJT based circuit as mentioned by you works as expected.

 

[FvM]

I don't agree that AD844 is a CCII, it's a regular current mode amplifier. Both have similar input but different output stages.

As long as the X-terminal is grounded (test circuit in post #1), the CCII can be replaced by an OTA.

 

[LvW]

I don't agree that AD844 is a CCII, it's a regular current mode amplifier. Both have similar input but different output stages.

FvM, sorry - but the AD844 is, indeed, a CCII+. (Also mentioned in the data sheet).
However, it has - in addition - a last stage onboard which acts as an impedance converter. This may lead to the false assumption that it is "only" a Current Feedback Amplifier (CFA).
But the high impedance node is available for CC applications.
I am aware that many, many papers have been published describing AD844 aplications with a title that refers to a "current-feedback amp". Of course, the AD844 can be used as a CFA, but the majority of the mentioned papers use the device as a CC (unfortunately, without mentioning).

 

[FvM]

I apologize for my persistence in this point. AD844 operation is based on an internal current conveyor, but as far as I understand it can't be used as such because the current source output isn't externally connected. As shown in the datasheet:

 

[LvW]

I apologize for my persistence in this point.....

The same applies to me.
I don`t know where the shown circuit diagram comes from.
Definitely, I know - as mentioned in my last post - that the high impedance node is available to connect external elements.
Many, many papers are published using this transimpedance z-node to realize CCII applications.

EDIT: According to the AD data sheet it is pin 5 (called TZ).

 

[FvM]

The diagram is from the AD844 datasheet. As many other schematics therein, it omits the TZ node. This made me overlook the CCII features of the device.

But you are completely right, the high impedance node is exposed as "TZ", and the internal load impedance is sufficiently high to use the device as a current conveyor.

 

[LvW]

....... the internal load impedance is sufficiently high to use the device as a current conveyor.

Yes - I have used (hardware !) this device as a CC in some filter and oscillator applications. As I have mentioned already, there are many papers describing various applications for the AD844 - mostly filters.
(Surprisingly, the authors in most cases speak about "current-feedback-amplifier" cicuits, often without mentioning the CC feature that is exploited).
However, I am not yet completely convinced that these new structures (or some of them) really have advantages if compared with "classical" filter/oscillator topologies.
One advantage could be the fact, that circuits are available with grounded capacitors only.

 

[FvM]

As far as I'm aware of, there's no specification in the datasheet about current output node TZ, except for Rt and Ct. No information about current transfer ratio, linear voltage range, bandwidth. Obviously, the datasheet focusses on the regular CFA application of AD844.

Another interesting CCII device from major manufacturers is OPA860/861.