BJT saturation

Mar 25, 2013 30 Replies

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But you didn't know that an inverted transistor does it better, which was common knowledge back in the 1970s.

Obviously, you can bodge the saturation voltage down to zero, but the crucial point is that the offset voltage of a saturated inverted transistor is a lot less than that of the same transistor biased conventionally. I've no idea whether the bodge current for exactly zero offset would be more or less, but you start off with less offset to cancel out.

One more case where you weren't paying enough attention as an undergraduate.

I got an extended lesson in the technique back in 1974, when my boss at Kent Instruments in Luton had me put together a slow but cheap and accurate two-quadrant PWM-based multiplier, where we used two saturating switches in series to get a very low offset voltage when the swtiches were on. The first transistor was run conventionally, and sank the bulk of the current, while the second was run inverted to minimise the offset voltage. Slow, but 0.1% accurate.

Bill Sloman, Sydney
[snip]
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How do you "bodge" the saturation voltage down to zero? Inquiring minds want to know ;-) ...Jim Thompson

-- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at

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| 1962 | I love to cook with wine. Sometimes I even put it in the food.

I just explained how, you idiot.

John Larkin Highland Technology, Inc jlarkin at highlandtechnology dot com http://www.highlandtechnology.com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom laser drivers and controllers Photonics and fiberoptic TTL data links VME thermocouple, LVDT, synchro acquisition and simulation

be saturated if increasing Ib no longer increases Ic. For example, if I have a circuit where the the collector of NPN is at +5V through a 1K resistor, maximum Ic is 5ma. Assuming hfe is 100, above 50uA of Ib doesn't increase Ic due to 1K resistor. Is this the saturation point for the transistor? If this is the case

something here?

below

At (the original) Fairchild, the C-B forward bias had to equal the E-B forward bias before we considered the transistor to not be active.

be saturated if increasing Ib no longer increases Ic. For example, if I have a circuit where the the collector of NPN is at +5V through a 1K resistor, maximum Ic is 5ma. Assuming hfe is 100, above 50uA of Ib doesn't increase Ic due to 1K resistor. Is this the saturation point for the transistor? If this is the case

something here?

the

below

Physically unrealizable _unless_ it's a symmetrical transistor, C-B junction identical to E-B. VERY uncommon. ...Jim Thompson

| James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.

be saturated if increasing Ib no longer increases Ic. For example, if I have a circuit where the the collector of NPN is at +5V through a 1K resistor, maximum Ic is 5ma. Assuming hfe is 100, above 50uA of Ib doesn't increase Ic due to 1K resistor. Is this the saturation point for the transistor? If this is the case

something here?

the

below

Wouldn't that require, for an NPN with grounded emitter, that the collector voltage be zero?

John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators

saturated if increasing Ib no longer increases Ic. For example, if I have a circuit where the the collector of NPN is at +5V through a 1K resistor, maximum Ic is 5ma. Assuming hfe is 100, above 50uA of Ib doesn't increase Ic due to 1K resistor. Is this the saturation point for the transistor? If this is the case

something here?

A more easily observable definition:

As you increase the base drive, the collector voltage falls to some minimum.

However, as you continue to increase the base drive, the collector voltage rises again beyond that minimum. That is full saturation. ...Jim Thompson

| James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.

to be saturated if increasing Ib no longer increases Ic. For example, if I have a circuit where the the collector of NPN is at +5V through a 1K resistor, maximum Ic is 5ma. Assuming hfe is 100, above 50uA of Ib doesn't increase Ic due to 1K resistor. Is this the saturation point for the transistor? If this is the case

missing something here?

the

below

Sounds like, by that criterion, no grounded-emitter transistor will ever be saturated.

John Larkin Highland Technology, Inc jlarkin at highlandtechnology dot com http://www.highlandtechnology.com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom laser drivers and controllers Photonics and fiberoptic TTL data links VME thermocouple, LVDT, synchro acquisition and simulation

be saturated if increasing Ib no longer increases Ic. For example, if I have a circuit where the the collector of NPN is at +5V through a 1K resistor, maximum Ic is 5ma. Assuming hfe is 100, above 50uA of Ib doesn't increase Ic due to 1K resistor. Is this the saturation point for the transistor? If this is the case

something here?

to be saturated if increasing Ib no longer increases Ic. For example, if I have a circuit where the the collector of NPN is at +5V through a 1K resistor, maximum Ic is 5ma. Assuming hfe is 100, above 50uA of Ib doesn't increase Ic due to 1K resistor. Is this the saturation point for the transistor? If this is the case

missing something here?

the

below

Check, but why limit to the NPN?

be saturated if increasing Ib no longer increases Ic. For example, if I have a circuit where the the collector of NPN is at +5V through a 1K resistor, maximum Ic is 5ma. Assuming hfe is 100, above 50uA of Ib doesn't increase Ic due to 1K resistor. Is this the saturation point for the transistor? If this is the case

something here?

I picked a randumb NPN and that "reversal" is seen in the region of a forced beta of 0.1 and Ic in the region of

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