Why these trimmer pots? Bias adjustment?

[dbanner]

Well I tried ltspice sim of the tx oscillator, the thing won't oscillate. Looking at the schematic, there's no dc bias resistor on the base of the npn transistor.

Luckily I have some photos of the PCB, so I'll try to see what's up with the schematic.

This circuit s a 13.77kHz machine.

[dbanner]

Thanks Carl. I'm going to work on this circuit some more, hopefully I can make a working machine out of it.

[Carl-NC]

Opamp offset is statistical, sometimes it adds, sometimes it subtracts. So you always assume it adds. The gain of these stages (they are the motion filter stages) is around 120, other designs may have filters with low gain. For this design, you need both low Vos and low Ib. The AD8609 would work, probably there are others.

[dbanner]

Question, if a non inverting op amp is followed by an inverting op amp, do the offset errors cancel? I see other schematics using this topology for this section of the circuit, but without offset trimmers. They are using common TL072 chips, nothing special.

[dbanner]

Thanks for explaining that. So it's an offset adjustment which trims out the input offset voltage error (LF444 datasheet indicates a Vos of 3mV minimum to 10mV) The TL074 is just about the same.

I like to go over these old schematics, they contain a lot of tricks the young players like myself (maybe not so young) can learn from, like the two series led's in the power supply.

[Carl-NC]

The first 2 trim out the offsets of U3d & U5b. So if you use opamps with really low Vos those can go. The other, I'm not sure. It's an offset adjust, but not sure why. U5a appears to be a rectifier.

That type of power supply is very common in VLF designs. Many detectors are now +5V with a rail splitter to create a +2.5V "analog ground."

[dbanner]

Can anyone shed light on the use of such trimmer pots as indicated in this schematic. This is an old design, but it is an interesting schematic.
Could these trimmers be eliminated altogether with a redesign of this section? This part of the schematic is using non-inverting inputs throughout.

Also the power supply (+6V +3V 0V-battery negative)is different than the traditional positive and negative rail voltages, with +3V serving as a virtual ground.