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Yaesu Incorrect Op-Amp Termination

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w3amd:
Hi all,

Is this an incorrect implementation of unused op-amp termination in Yaesu radios? I'm only a novice. I think this is probably incorrect but want to ask the experts.



Cheers,
John

ciccio:
I've seen this done in many devices. The first time I've seen it (I was repairing it) it I've checked the circuit for instability, and could not find none.
The book you are referring to says NO to the the solution I always use, that is output shorted to inverting pin and non-inverting pin grounded.
I've done this from 1970's, without problems, NEVER any problem.
I generally use dual supplies, and when I have a single supply the non-inverting input is wired to virtual ground (1/2 supply voltage) and not to ground.
My experience is with audio amplifiers and DC amplifier used in industrial controls. Other application may have different requirements, but I doubt.
Maybe the book refer to single supply amplifiers.... or the book assertions came from other books, and not from direct experience.

Best regards

T3sl4co1l:
Depends on type.  Grounding inputs is very common.  Or VEE, or VCC or something inbetween.  Or a skewed state (one input above the other, so the output is fixed high or low), or a follower connection (+in = VREF, -in = out) so it can do its job as usual (i.e., amplifying with feedback, you're just not using the signal).

Incorrect setting can cause problems, because multi-part amps often share bias circuitry.  Causing one section's circuitry to saturate (inputs exceeding common mode range, output saturating?) can cause problems in the others.  This is usually not a problem on modern parts, but RTFDS to be sure.

Tim

w3amd:

--- Quote from: T3sl4co1l on December 27, 2015, 01:06:15 pm ---Depends on type.  Grounding inputs is very common.  Or VEE, or VCC or something inbetween.  Or a skewed state (one input above the other, so the output is fixed high or low), or a follower connection (+in = VREF, -in = out) so it can do its job as usual (i.e., amplifying with feedback, you're just not using the signal).

Incorrect setting can cause problems, because multi-part amps often share bias circuitry.  Causing one section's circuitry to saturate (inputs exceeding common mode range, output saturating?) can cause problems in the others.  This is usually not a problem on modern parts, but RTFDS to be sure.

Tim

--- End quote ---

Thanks Tim. I'm going to try to take a look at the chip they used in both radios and see what the common mode specs are.

Here is another video from Texas Instruments itself:


The datasheet for the op-amp in the FT-450D radio is an LM2904:
http://www.ti.com/lit/ds/symlink/lm258a.pdf

Cheers,
John

SeanB:
Page12 shows a schematic, showing the input stage is a PNP transistor,. so it is fine being grounded. If the input common mode does not include ground you need a simple voltage somewhere inside the common mode range to connect to the one input and simply use the opamp as a buffer if it is stable as a unity gain buffer, otherwise you will have to use a minimum gain configuration to keep it stable.

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