# Circuit to pulse from 1v to 1.1v repeatedly.

• 21st July 2019, 21:04
treez
Circuit to pulse from 1v to 1.1v repeatedly.
Hi ,
We need the “Output”of the following circuit to go from 1v to 1.1v as shown in the LTspice simulation. We are worried we may get too much VCE drop in the pnp….the datasheet does not tell the vce drop for our conditions, but can you agree that our Vce will be less than 10mV when ON?

2N3906
https://www.onsemi.com/pub/Collateral/2N3906-D.PDF
• 21st July 2019, 21:17
KlausST
Re: Circuit to pulse from 1v to 1.1v repeatedly.
Hi,

Why no Mosfet?

Klaus
• 21st July 2019, 21:28
treez
Re: Circuit to pulse from 1v to 1.1v repeatedly.
Thanks......it would need to be pfet, and i cant pull the gate down far enough as its only a 1.1v rail.
(admittedly we could make a negative rail for this purpose but its extra components)
• 21st July 2019, 21:45
betwixt
Re: Circuit to pulse from 1v to 1.1v repeatedly.
Wouldn't it be more sensible to switch the voltage at the input of the op-amp where the current is small and independent of load. You can set the voltage with resistors, use the op-amp output directly and avoid the transistors altogether.

Brian.
• 21st July 2019, 22:09
FvM
Re: Circuit to pulse from 1v to 1.1v repeatedly.
Quote:

can you agree that our Vce will be less than 10mV when ON?
Not sure, at least several mV.

To implement a BJT analog switch with zero offset voltage, operate the transistor inversely. Disadvantage is current gain of about unity.
• 22nd July 2019, 03:07
asdf44
Re: Circuit to pulse from 1v to 1.1v repeatedly.
Quote:

Originally Posted by betwixt
Wouldn't it be more sensible to switch the voltage at the input of the op-amp where the current is small and independent of load. You can set the voltage with resistors, use the op-amp output directly and avoid the transistors altogether.

Brian.

Agree: just switch or mux the opamp reference.

A second option is to mux two different sources. NLASB3157 is an example of SPDT analog mux.
• 22nd July 2019, 14:04
Re: Circuit to pulse from 1v to 1.1v repeatedly.
From a 1V supply the astable multivibrator produces 1.35V peaks at the capacitors.
I'm not sure if this can be adapted to your purposes but the topic appears to ask for a simple method to get slightly higher voltage from a 1V source.

Attachment 154536

My schematic has PNP type. The usual schematic has NPN type and so it produces negative polarity at the capacitors.
• 23rd July 2019, 02:56
Easy peasy
Re: Circuit to pulse from 1v to 1.1v repeatedly.
from the graphs in the data sheet - the best you can do with this device is about 80mV for 1mA collector and 100uA base, there are a heap better pnp's for this, usually 1 amp from zetex ...
• 23rd July 2019, 15:29
FvM
Re: Circuit to pulse from 1v to 1.1v repeatedly.
Circuit load current wasn't yet specified. Even low Vcesat transistors (e.g. NXP "BISS" types) have Vcesat almost converging against a constant value at low Ic values. Vcesat is caused by forward biased BC diode and it's voltage characteristic related to BE diode.

Attachment 154555

As previously mentioned, you get < 1mV Vcesat in inversed operation, no idea if it helps for your application.