# Bipolar transistor bias question

Started by December 18, 2016
```I have a switching application where a single bipolar transistor might be
used. The AC input is about 300mV and the transistor needs about 700 mV to
switch on. My idea is to construct a voltage divider using a diode which
will produce about a 600mV drop across the diode and a voltage divider of 2
equal resistors to set the transistor base voltage at about 300 mV so the
transistor will switch on with an additional 300mV. I understand the
temperature problems, but it seems to be minimal. What am I missing?

```
```On Monday, 19 December 2016 00:41:55 UTC, Bill Bowden  wrote:
> I have a switching application where a single bipolar transistor might be
> used. The AC input is about 300mV and the transistor needs about 700 mV to
> switch on. My idea is to construct a voltage divider using a diode which
> will produce about a 600mV drop across the diode and a voltage divider of 2
> equal resistors to set the transistor base voltage at about 300 mV so the
> transistor will switch on with an additional 300mV. I understand the
> temperature problems, but it seems to be minimal. What am I missing?

a circuit diagram.

NT
```
```On Sun, 18 Dec 2016 16:41:42 -0800, "billbowden"
<bperryb@bowdenshobbycircuits.info> wrote:

>I have a switching application where a single bipolar transistor might be
>used. The AC input is about 300mV and the transistor needs about 700 mV to
>switch on. My idea is to construct a voltage divider using a diode which
>will produce about a 600mV drop across the diode and a voltage divider of 2
>equal resistors to set the transistor base voltage at about 300 mV so the
>transistor will switch on with an additional 300mV. I understand the
>temperature problems, but it seems to be minimal. What am I missing?
>
>
>
>

More detail would help. What is the drive waveform like? What is the
load? Got a schematic?

A PNP emitter follower can be a neat way to drive an NPN transistor.
It moves the effective threshold near zero volts and does some
temperature compensation.

Or just add an IC, an opamp or a comparator.

--

John Larkin         Highland Technology, Inc

lunatic fringe electronics

```
```On 12/18/2016 7:41 PM, billbowden wrote:
> I have a switching application where a single bipolar transistor might be
> used. The AC input is about 300mV and the transistor needs about 700 mV to
> switch on. My idea is to construct a voltage divider using a diode which
> will produce about a 600mV drop across the diode and a voltage divider of 2
> equal resistors to set the transistor base voltage at about 300 mV so the
> transistor will switch on with an additional 300mV. I understand the
> temperature problems, but it seems to be minimal. What am I missing?

Why not use a Schottky diode with it's lower voltage drop eliminating
the need for the divider resistors?

I think the schematic would be like this.  Size the resistor to give the
voltage drop required when the input is at 0.3 volts.

Vcc
___
|
\
/      +---- Out
\      |
|    |/
-->>---S<|---+----|
|\|
\
|
|
-----
---
-
--

Rick C
```
```On Sun, 18 Dec 2016 16:41:42 -0800, "billbowden"
<bperryb@bowdenshobbycircuits.info> wrote:

>I have a switching application where a single bipolar transistor might be
>used. The AC input is about 300mV and the transistor needs about 700 mV to
>switch on. My idea is to construct a voltage divider using a diode which
>will produce about a 600mV drop across the diode and a voltage divider of 2
>equal resistors to set the transistor base voltage at about 300 mV so the
>transistor will switch on with an additional 300mV. I understand the
>temperature problems, but it seems to be minimal. What am I missing?
>
>

Fundamentals.

...Jim Thompson
--
| James E.Thompson                                 |    mens     |
| Analog Innovations                               |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| STV, Queen Creek, AZ 85142    Skype: skypeanalog |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
```
```On Sun, 18 Dec 2016 16:41:42 -0800, billbowden wrote:

> I have a switching application where a single bipolar transistor might
> be used. The AC input is about 300mV and the transistor needs about 700
> mV to switch on. My idea is to construct a voltage divider using a diode
> which will produce about a 600mV drop across the diode and a voltage
> divider of 2 equal resistors to set the transistor base voltage at about
> 300 mV so the transistor will switch on with an additional 300mV. I
> understand the temperature problems, but it seems to be minimal. What am
> I missing?

You may be pushing what can be done (reliably) with a single transistor.
When you provide us with more information (as others have suggested) be
sure to include the collector circuit - or at least a more solid
definition of what "on" and "off" are.
```
```>"What am I missing?"

The fact that you should just build a friggin drive circuit for it.

You use a diode to up a 300 mV drive to positive 700 mV to turn the transistor on, first of all it might not be enough to saturate it. And then when it drops off it is probably not going to turn it off very fast.

If you are dealing with an inductive load this is poison. The Vce rises so fast it results in so much power dissipation it is almost not worth using a switcher.

In an SMPS with a bipolar, usually there is some drive enhancements, some of which drive the base reverse by a few volts to get the thing to turn off really fast, if the load is highly inductive, but you did not say it is. However a switcher is hard to use without an inductor, at least effectively when it comes to efficiency.

So just take your 300 mV and amplify it, that'll work. One simple little transistor and a few resistors should do it, if it is some sort of self oscillating nightmare, then it takes two transistors.

Are there space constraints ? If it is cost tell the boss "You want to issue refunds to ALL your customers ?". Another workaround in some applications is just to use a transistor that costs ten times as much, and they won't like that either.

Any solutions, they will depend on the scale. How much voltage and current ?
```
```billbowden <bperryb@bowdenshobbycircuits.info> wrote:
> I have a switching application where a single bipolar transistor might be
> used. The AC input is about 300mV and the transistor needs about 700 mV to
> switch on. My idea is to construct a voltage divider using a diode which
> will produce about a 600mV drop across the diode and a voltage divider of 2
> equal resistors to set the transistor base voltage at about 300 mV so the
> transistor will switch on with an additional 300mV. I understand the
> temperature problems, but it seems to be minimal. What am I missing?

You are not providing enough detail.  What kind of current is involved,
what is the impedance of the source providing the 300mV, etc.

Transistors are switched mainly by current, not voltage.   Your source
will need to provide the driving current of the transistor, and it
cannot be told from "it is 300mV" whether it will be able to do that.
```
```On Sun, 18 Dec 2016 16:41:42 -0800, "billbowden"
<bperryb@bowdenshobbycircuits.info> wrote:

>I have a switching application where a single bipolar transistor might be
>used. The AC input is about 300mV and the transistor needs about 700 mV to
>switch on. My idea is to construct a voltage divider using a diode which
>will produce about a 600mV drop across the diode and a voltage divider of 2
>equal resistors to set the transistor base voltage at about 300 mV so the
>transistor will switch on with an additional 300mV. I understand the
>temperature problems, but it seems to be minimal. What am I missing?
>
>

<http://www.analog-innovations.com/SED/ThresholdAmp_Bowden_SED_2016-12-19.png>

(Pardon my snarky response last night... I fell asleep watching a
movie on TV and didn't read your post carefully :-)

...Jim Thompson
--
| James E.Thompson                                 |    mens     |
| Analog Innovations                               |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| STV, Queen Creek, AZ 85142    Skype: skypeanalog |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
```
```On Mon, 19 Dec 2016 09:32:14 -0700, Jim Thompson
<To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote:

>On Sun, 18 Dec 2016 16:41:42 -0800, "billbowden"
><bperryb@bowdenshobbycircuits.info> wrote:
>
>>I have a switching application where a single bipolar transistor might be
>>used. The AC input is about 300mV and the transistor needs about 700 mV to
>>switch on. My idea is to construct a voltage divider using a diode which
>>will produce about a 600mV drop across the diode and a voltage divider of 2
>>equal resistors to set the transistor base voltage at about 300 mV so the
>>transistor will switch on with an additional 300mV. I understand the
>>temperature problems, but it seems to be minimal. What am I missing?
>>
>>
>
><http://www.analog-innovations.com/SED/ThresholdAmp_Bowden_SED_2016-12-19.png>
>
>(Pardon my snarky response last night... I fell asleep watching a
>movie on TV and didn't read your post carefully :-)
>
>                                        ...Jim Thompson

If the input can be AC coupled, this might work:

+5       whatever
|          |
|          |
|          |
R          |
|          |
|          c
in------C---------+---------b
e
|
gnd

It's automagically self-biasing.

--

John Larkin         Highland Technology, Inc

lunatic fringe electronics

```