# Open collector buffers VS Non Open Collector Buffers

Discussion in 'General Electronics Chat' started by DexterMccoy, Mar 6, 2014.

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1. ### DexterMccoy Thread Starter New Member

Feb 19, 2014
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What's the difference between Open collector buffers VS Non Open Collector Buffers?

For logic circuits , using an Open Collector buffer, it should output a low state voltage and a high state voltage

For logic circuit, using a Non open collector buffer, it outputs a low state voltage and a high state voltage

What is the difference between the two buffers low state voltage and high state voltage?

An open collector buffer needs a pull up resistor? VS a non open collector buffer doesn't need a pull up resistor.

What difference does having the pull up resistor do to the buffer and logic high and low states?

2. ### Efron Member

Oct 10, 2010
81
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In an open collector buffer, you need a pull up resistor in order to give you a HIGH level output.

With this pull up resistor, you've got two possible output levels: LOW (e.g. 0V) and HIGH (e.g. Vcc).

If you don't use any pull up resistor, you've got two possible output levels: LOW (e.g. 0V) and HIGH IMPEDANCE.

The High Impendance comes from the fact that the output phase of the open collector buffer is done by a Bipolar transistor or Mosfet where the collector/drain is left floating (not connected to anything).

In the case of the bipolar technology, when the base-emitter of the output transistor is forward polarized, the transistor becomes like a very low value resistor between its collector (output pin) and its emitter (e.g. 0V). In this case you've got a LOW level output.

However, if it is not polarized, the resistor seen between the output pin and the emitter is High Impendance (can be more than 100MEG).

By using a pull up resistor, you convert the output to HIGH level (e.g. Vcc).

The use or not of a pull up resistor depends on the application.

Jul 18, 2013
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One feature of open collector outputs is they can be OR'ed when connected to one common load.
Max.

4. ### inwo Well-Known Member

Nov 7, 2013
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Another advantage is interfacing or driving a higher voltage than supply.

5. ### DexterMccoy Thread Starter New Member

Feb 19, 2014
429
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What you mean by interfacing? interfacing different voltages together?

I know buffers are used to do level shifting which converts a logic high state into a higher logic high stage or other voltages

6. ### DexterMccoy Thread Starter New Member

Feb 19, 2014
429
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I do see them tied the outputs of open collector buffers to increase the current, so it drives the logic chips more

I'm guessing the push pull buffers don't drive high or have a low current output

7. ### DexterMccoy Thread Starter New Member

Feb 19, 2014
429
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Any reasons why you would want to increase the Drive current? for what reasons or logic advantage?

8. ### DexterMccoy Thread Starter New Member

Feb 19, 2014
429
2
Here is the schematics

The 2004 Buffers added in parallel increase the Current higher to drive Lamp lights and Coils of Relays

My question why didn't they use open collector buffers?

When do you see buffers like this being added in parallel to increase the current higher, for what? besides for driving lamps and coils to relays?

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9. ### ErnieM AAC Fanatic!

Apr 24, 2011
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Towards what end do you post the same section of a schematic three times in the same post?

10. ### DexterMccoy Thread Starter New Member

Feb 19, 2014
429
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U27 & U33 is a 2004 buffer, connecting the outputs together, but this a non-open collector buffer

Mostly you only see Open collector buffers have their output pins tied together

My question is why did they use a Non-open collector buffer? plus no pull up resistors on the output pins

What does this do differently

11. ### MrChips Moderator

Oct 2, 2009
12,657
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Impedance, impedance, impedance.

Your questions are separate and don't have anything to do with each other.

1) Why did they tie three buffers in parallel with each other?

Impedance. The CMOS 4011 gate has relatively high output impedance and cannot drive a low impedance load. The three buffers tied together provide low impedance output allowing it to be able to drive a low impedance load.

2) Totem-pole output gates (non-open-collector outputs) can source or sink current. No additional output resistor is required.

3) Open-collector outputs are exactly that. The collector output is uncommitted and gives the circuit designer the flexibility to choose his/her own output voltage and current. The output transistor with open collector can sink current but cannot supply (source) current. The supply (source) current has to be provided with an external pull-up resistor.

Edit: Why did the circuit designer choose 2004 buffer? Who knows.
Another solution would have been to use a BJT or MOSFET to drive the external loads.

Last edited: Mar 21, 2014
12. ### Little Ghostman Member

Jan 1, 2014
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Thanks I think you have just explained why someone I know said that a transistor was infact a TRANSFORMING RESISTOR, I didnt have a clue what he was on about, now I get it (obvious I guess when its spelt out lol)

13. ### JWHassler Member

Sep 25, 2013
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It seems to me that the symbols on the '2004' buffers in those four schematics DO indicate that they are open-collector. The little extra bar on the triangle's nose is one of the ways of showing that

14. ### DexterMccoy Thread Starter New Member

Feb 19, 2014
429
2
How can they be open collector when there is no pull up resistors? Unless the load is the pull up resistor?

What can some low impedance loads? What are some high impedance loads? Is a relay coil a high or low impedance load? Is a lamp a low or high impedance load?

Are open collector buffers used for low or high impedance loads? And buffers that don't use a pull up resistor are for low or high impedance loads?

Last edited by a moderator: Mar 21, 2014
15. ### MrChips Moderator

Oct 2, 2009
12,657
3,461
You are mixing your questions again.

Open collector and the impedance of the load are not necessarily related.

TTL gates have limited drive capability. They can sink 8-20mA depending on the TTL series. They are not good for supplying current. CMOS gates are not any better for source or sink.

Open collector gives the circuit designer many more output options:

1) Choice of output voltage,
2) Choice of source current,
3) Ability to perform wired-OR logic function

Open collector buffers can be used for both high impedance loads and low impedance loads, up to a point. The devil is in the details.

A relay coil or lamp would be considered a low impedance load. But that is a relative statement depending on the context. In terms of logic outputs, I would loosely say a load that takes less than 1mA is high impedance and a load that takes more than 10mA is low impedance.

NOTE: this is not a definitive statement and it depends on the specific situation. In another situation, 100kΩ load could be regarded as low impedance if the driver was expected to drive a 1MΩ load.