Small voltage detector

you'll need 2 transistors, a PNP emitter follower to raise the voltage enough for an NPN to turn on. (this is how LM319 includes ground in the input range)

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Are you allowed to bias the base positive? That is, can the voltage you want to measure sink as well as supply?

Forgot to add an illustration:

VCC + .|. | | | | '-' | | ___ | |/ In -|___|- -+------| |> | | === GND (created by AACircuit v1.28.6 beta 04/19/05

Say the input is the secondary of a power transformer (which is my actual application, but I'll use a built-in comparator module in the real circuit). So, assume I can.

Best regards, Piotr

Something like this (assuming the source can drive 1k)?

Not particularly stable, but may work at room temperature...

Dimitrij

Germanium transistor?

Regards,

Boris Mohar

Got Knock? - see: Viatrack Printed Circuit Designs (among other things)

void _-void-_ in the obvious place

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Voltages outputed from the secondary of a power transformer are typically bipolar, low impedance and large, so your description of the measurement needing to be made doesn't make much sense.

Are you trying to detect a zero crossing? a change in current flow? What?

Simply offseting the base bias by a diode drop will provide first order compensation for a common base amplifier with high voltage gain, if the source impedance is low.

The use of any amplifier as a comparator will always have issues.

Low power, single supply comparators are avilable in 5 or 6pin transistor-sized packaging that will operate with inputs at 100s of millivolts either side of ground.

RL

that's the simplest option afaik. You might improve it by adding a series diode in the input, then Rin can be lower. You may also need a 2nd diode to protect against Veb >5v.

NT

Good observation.

Long tail pair with a diode reference?

That's it. Two transistors and one resistor make a zero-threshold open-drain comparator.

After the bridge the transformer's voltage is unipolar and somewhat shifted (by a diode drop if it is a normal silicon Graetz circuit, way smaller if synchronous).

It does, the result indicates which branch of the H bridge should be turned on in a synchronous rectifier. I wondered how would a discrete-only analog controller work if the switch enable threshold is required to be low. My first guess was to clone the input differential pair from an op-amp, but it would be too complex. So I decided to ask the analog gurus here and Jasen's solution needs only 2 transistors, which is much simpler than my idea.

Yes, and they are also integrated in the MCUs, i.e. no external elements (other than a voltage divider perhaps) are needed, so the question is purely theoretical.

Best regards, Piotr

Thanks to all who answered my question, I've learnt several new tricks. :-)

Best regards, Piotr

;-)

The question is really about simple sub-threshold voltage detection techniques and it on purpose ignores the fact that we live in the 21st century, where the problem has already been solved by cheap solid-state integrated circuits in nice packages. Currently I like the PNP follower idea most

-- just three parts and pretty universal.

Best regards, Piotr

discrete is cheaper here, albeit lower spec.

NT

Somebody makes a mosfet with a zero volt threshold. That and a drain pullup is the minimal discrete zero-sense comparator.

One problem: as the voltage goes down, the required gain goes up (or so one would assume). This is especially important in a comparator (ideally infinite GBW).

So you inevitably need more transistors, or better (GaN maybe??).

Three works pretty well:

The pair needs to be heavily unbalanced to actually include zero (note the

2.2k is supplying about 6mA, while the gain transistor needs maybe 0.2mA to turn on), but that isn't usually a problem for this type of application. Same for the poor turn-off (the gain transistor will take relatively long storage time here, but that actually helps in this case).

Tim

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"Piotr Wyderski"  wrote in message  
news:muo6o0$mu6$1@node2.news.atman.pl... 

Hello, 

I know that there are hordes of nice comparators in SOT-23, 
but just out of curiosity: is it possible to build a small 
voltage (relatively to GND) detector using just one transistor 
and no negative power supplies? If the voltage is >= VBE, i.e. 
0.7V, then the task is a no-brainer, but how about half of that, 
say, 0.3V? 

Best reagrds, Piotr

I don't see how that would be true. Only the gain at & near threshold V matters, and the amount of gain required depends entirely on the circuit the threshold detector drives. Delta V matters, not V.

No

NT