Transistor base as a diode

G

Gazza

Guest
Hello,

I have seen several schematics where a transistor base has been used as
a diode where the base is also short circuited to the collector (NPN).
Please can anybody tell me why this is done and what the benefits are?

Many thanks in advance.

Gary
 
In article <1164796423.575546.137960@h54g2000cwb.googlegroups.com>,
gary_windsor@hotmail.com says...
I have seen several schematics where a transistor base has been used as
a diode where the base is also short circuited to the collector (NPN).
Please can anybody tell me why this is done and what the benefits are?
Click to expand...
The most common case that comes to mind is a current mirror circuit,
where you need an input diode with essentially the same I/V curve as
your output transistor.
 
ian field wrote:

"Bill S." <NoSpam@OnMe.com> wrote in message
news:MPG.1fd795bc37332b86989694@news-server.woh.rr.com...

In article <1164796423.575546.137960@h54g2000cwb.googlegroups.com>,
gary_windsor@hotmail.com says...

I have seen several schematics where a transistor base has been used as
a diode where the base is also short circuited to the collector (NPN).
Please can anybody tell me why this is done and what the benefits are?

The most common case that comes to mind is a current mirror circuit,
where you need an input diode with essentially the same I/V curve as
your output transistor.


The way I read that, is as the OP mentioning a transistor with its CB
shorted and the BE used as a diode - as opposed to in a configuration such
as a CM. One application that springs to mind is that it yields a slightly
lower Vf than a regular diode, the collector will begin to draw current at a
VBE of around 0.5V. This is also very dependent on temperature, I recall
that the old Germanium AC128 was often used in this way as a temperature
compensator in the convergence panels of old delta-gun CTVs. Also most Si
transistors exhibit a zener voltage in reverse - usually around 5V or so.
Click to expand...
Sometimes the reason is much more mundane: Logistics. If the board
doesn't need many diodes but already has transistors then using a
transistor instead can reduce the number of BOM line items by one. Saves
inventory costs, rigging costs etc.

--
Regards, Joerg

http://www.analogconsultants.com
 
ian field wrote:

"Bill S." <NoSpam@OnMe.com> wrote in message
news:MPG.1fd795bc37332b86989694@news-server.woh.rr.com...
In article <1164796423.575546.137960@h54g2000cwb.googlegroups.com>,
gary_windsor@hotmail.com says...
I have seen several schematics where a transistor base has been used as
a diode where the base is also short circuited to the collector (NPN).
Please can anybody tell me why this is done and what the benefits are?

The most common case that comes to mind is a current mirror circuit,
where you need an input diode with essentially the same I/V curve as
your output transistor.

The way I read that, is as the OP mentioning a transistor with its CB
shorted and the BE used as a diode - as opposed to in a configuration such
as a CM.
That is the same configuration, for the input side of the current mirror.
Click to expand...

One application that springs to mind is that it yields a slightly
lower Vf than a regular diode, the collector will begin to draw current at
a VBE of around 0.5V. This is also very dependent on temperature, I recall
that the old Germanium AC128 was often used in this way as a temperature
compensator in the convergence panels of old delta-gun CTVs. Also most Si
transistors exhibit a zener voltage in reverse - usually around 5V or so.
Click to expand...
But if you ever make them conduct in reverse like this (as a Zener) then
those devices should be treated as suspect parts from then on. It has been
alleged to do funny things to the noise performance etc. Certainly on some
IC processes it is frowned upon to let that junction break down, even at
low current levels.

Chris
 
Gazza wrote:
Hello,

I have seen several schematics where a transistor base has been used as
a diode where the base is also short circuited to the collector (NPN).
Click to expand...
The logarithm characteristics of a perfect diode are shared by this
kind
of connected transistor; so-called 'ideal' diode behavior is used in
logarithm
converters, and few 'switch' diodes (which are optimized for low
capacitance) or
rectifier diodes (which are optimized for high currents) can compete
with
a good low-noise transistor.
 
"Bill S." <NoSpam@OnMe.com> wrote in message
news:MPG.1fd795bc37332b86989694@news-server.woh.rr.com...
In article <1164796423.575546.137960@h54g2000cwb.googlegroups.com>,
gary_windsor@hotmail.com says...
I have seen several schematics where a transistor base has been used as
a diode where the base is also short circuited to the collector (NPN).
Please can anybody tell me why this is done and what the benefits are?

The most common case that comes to mind is a current mirror circuit,
where you need an input diode with essentially the same I/V curve as
your output transistor.
Click to expand...
The way I read that, is as the OP mentioning a transistor with its CB
shorted and the BE used as a diode - as opposed to in a configuration such
as a CM. One application that springs to mind is that it yields a slightly
lower Vf than a regular diode, the collector will begin to draw current at a
VBE of around 0.5V. This is also very dependent on temperature, I recall
that the old Germanium AC128 was often used in this way as a temperature
compensator in the convergence panels of old delta-gun CTVs. Also most Si
transistors exhibit a zener voltage in reverse - usually around 5V or so.
 
Gazza wrote:
I have seen several schematics where a transistor base has been used as
a diode where the base is also short circuited to the collector (NPN).
Please can anybody tell me why this is done and what the benefits are?
Click to expand...
This is sometimes called a "super diode"; it has a maximum current
rating equal to the maximum collector current of the transistor, and
tends to have a nice low forward voltage drop at moderate currents
compared with average small signal diodes, but the main advantage is
that the diode characteristics match the characteristics of other
transistors in the circuit so you get (for example) good current mirrors
, especially if both are part of the same IC, so good thermal tracking
and in some situations low distortion or little error over a wide
operating current range.

Mark A
 
Mark Aitchison wrote:

Gazza wrote:
I have seen several schematics where a transistor base has been used as
a diode where the base is also short circuited to the collector (NPN).
Please can anybody tell me why this is done and what the benefits are?

This is sometimes called a "super diode"; it has a maximum current
rating equal to the maximum collector current of the transistor, and
tends to have a nice low forward voltage drop at moderate currents
compared with average small signal diodes, but the main advantage is
that the diode characteristics match the characteristics of other
transistors in the circuit so you get (for example) good current mirrors
, especially if both are part of the same IC, so good thermal tracking
and in some situations low distortion or little error over a wide
operating current range.
Click to expand...
It avoids much of the IR voltage drop of using the b-e diode alone since most of
the current flows c-e.

Graham
 
Mark Aitchison wrote:

Gazza wrote:

I have seen several schematics where a transistor base has been used as
a diode where the base is also short circuited to the collector (NPN).
Please can anybody tell me why this is done and what the benefits are?


This is sometimes called a "super diode"; it has a maximum current
rating equal to the maximum collector current of the transistor, and
tends to have a nice low forward voltage drop at moderate currents
compared with average small signal diodes, but the main advantage is
that the diode characteristics match the characteristics of other
transistors in the circuit so you get (for example) good current mirrors
, especially if both are part of the same IC, so good thermal tracking
and in some situations low distortion or little error over a wide
operating current range.

Mark A
They make pretty good Zeners also if you need one around that voltage
Click to expand...
region that is heat sinkable! :)



--
"I'm never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5
 
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