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From: Précis d'electronique on 2 Jun 2010 22:50
Hi, I would like to know in your opinion when and why use resistors
between the base an the emitter in a transistor.


I'm making a bridge h but not to control a motor, it is for having a
+/- signal at a frequency>100Hz, I just have a resistor as a charge,
my signal is 200Vpp Iam considering to use MOSFETs.

my circuit its very similar to this one

http://www.mcmanis.com/chuck/Robotics/tutorial/h-bridge/images/bjt-schematic.gif

I simulate it but it seems (the signal) better without the resistors.

Thanks in advance.
From: Bill Sloman on 3 Jun 2010 05:22
On Jun 3, 4:50 am, Précis d'electronique <ee.jce...(a)gmail.com> wrote:
> Hi, I would like to know in your opinion  when and why use resistors
> between the base an the emitter in a transistor.
>
> I'm making a bridge h but not to control a motor, it is for having a
> +/- signal at a frequency>100Hz, I just have a resistor as a charge,
> my signal is 200Vpp Iam considering to use MOSFETs.
>
> my circuit its very similar to this one
>
> http://www.mcmanis.com/chuck/Robotics/tutorial/h-bridge/images/bjt-sc...
>
>  I simulate it but it seems (the signal) better without the resistors.

The base-emitter resistor carries the collector-base leakage current
when the transistor is off; without the resistor this current flows
into the base and is multiplied by the beta (current gain) of the
transistor.

It also speeds up turn-off. The collector-base (Miller) capacitance
will discharge much more slowly through the base-emitter junction than
through the 10k resistors shown.

--
Bill Sloman, Nijmegen

From: Jamie on 3 Jun 2010 18:21
Pr�cis d'electronique wrote:
> Hi, I would like to know in your opinion when and why use resistors
> between the base an the emitter in a transistor.
>
>
> I'm making a bridge h but not to control a motor, it is for having a
> +/- signal at a frequency>100Hz, I just have a resistor as a charge,
> my signal is 200Vpp Iam considering to use MOSFETs.
>
> my circuit its very similar to this one
>
> http://www.mcmanis.com/chuck/Robotics/tutorial/h-bridge/images/bjt-schematic.gif
>
> I simulate it but it seems (the signal) better without the resistors.
>
> Thanks in advance.
So they can drain any charge on the base to force it to turn off sooner?

Other wise, you can run into a problem of hanging transistors which
could cause them to be slightly in conductive state. This can cause some
heating of the bridge.. Especially as the freq increase..

This is just my guess and a good one I might add ;)

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