How does a buffer work?

[darksky1]

Hi, i'm an electronics noob.. anyway, i've been trying to understand buffers but i'm confused.. i found it hard to find links which describe in detail each part of the buffer..datasheets aren't helping me much

from the SN74LS240 buffer datasheet for example i see that each input has some kind of inverting opamp, is this for amplifying current? for ex. connecting a stepper motor to the output and microcontroller to the input?

it states this on the datasheet:

Features:
n 3-STATE outputs drive bus lines directly
n PNP inputs reduce DC loading on bus lines
n Hysteresis at data inputs improves noise margins
n Typical IOL (sink current)
24 mA
n Typical IOH (source current)
-15 mA
n Typical propagation delay times
Inverting 10.5 ns
Noninverting 12 ns
n Typical enable/disable time 18 ns
n Typical power dissipation (enabled)
Inverting 130 mW
Noninverting 135 mW


but i'm not really understanding any of this, could someone provide me with some links which will improve my understanding of buffers? would be appreciated

 

[audioguru]

Its output current is much too low for it to drive a stepper motor. It buffers logic signals that are only 4mA.
Your motor might draw hundreds of mA or maybe a few Amps.

 

[darksky1]

ok so it won't drive a stepper motor but never mind that, that was just a random example

**broken link removed**

look at the 'insides' of the buffer, what are these inside it? are they inverting opamps or some kind of transistors?

 

[ericgibbs]

hi,
Look here for one explanation.

 

[darksky1]

thanks for the reply

 

[koolguy]

how to make buffer using 741 Op amp??

 

[atferrari]

Here we go again...

 

[audioguru]

A 741 opamp is 43 years old and has very poor performance. It is designed for a 30V supply.
This thread talks about digital logic buffers that use a 5V supply, not old opamps.

 

[shortbus=]

how to make buffer using 741 Op amp??

1)Take your 741 opamp, smash it with a BIG hammer.

2) Go to the electronics parts store and buy a new buffer chip.

3) done.

 

[koolguy]

I am asking for basic circuit design..!!

 

[ericgibbs]

I am asking for basic circuit design..!!

Look here.

 

[audioguru]

The buffer IC was used for old 74xx TTL logic and old 74LSxx TTL logic so that outputs could drive more inputs.
You might not ever use old TTL ICs (I used them last about 25 years ago) and you might not never need to drive more inputs (I never did) since an un-buffered TTL logic part can drive up to 10 TTL inputs and an un-buffered LS-TTL part can drive up to 20 LS-TTL inputs.

When I stopped designing logic circuits with old TTL then I began using CD4xxx ordinary Cmos and 74HCxxx high-speed Cmos ICs that don't need buffers to drive more inputs.

 

[koolguy]

so, what the difference in inverting & non- inverting buffer??

 

[audioguru]

so, what the difference in inverting & non- inverting buffer??

A part that inverts has a high output when the input is low and has a low output when the input is high.
A part that does not invert has the same logic output as its input.

 

[killivolt]

A part that inverts has a high output when the input is low and has a low output when the input is high.
A part that does not invert has the same logic output as its input.

Excellent and easy to understand Mcquacky

Eric, this is the first time I've ever tried to understand logic and buffer very, very easy for this old guy to get it. (It was eureka moment)

I don't think I'll be designing anything soon but, I maybe able to follow discussions in other threads in the future.

Thank you.

Edit: Thanks also goes out the OP. Good Question.

 

[koolguy]

Hi, why we use buffers so much as there gain is 1, like modulators FM, AM,etc??

 

[audioguru]

An analog buffer has a high input impedance, a low output impedance and a voltage gain of 1.

 

[koolguy]

Ok, that mean voltage gain is 1, but current is variable.

 

[kpatz]

Buffers (in analog circuits) are used when you need to lower the impedance/increase current capability. They're also useful if you want to tap a signal in a circuit and don't want to put a load on that circuit that might cause the voltage to drop. For example, you might have a circuit that generates a signal and you want to bring that signal out to an external connector that you can plug a data logger or whatever into. If you put a buffer in between the original signal and the data logger output, you don't have to worry about the data logger causing a voltage drop on the original signal. Or if you accidentally short out the output, the circuit won't be affected or damaged.

Another use for a buffer is if you have a chip that has a high output impedance (such as an accelerometer) which you want to feed into a ADC that has a lower input impedance. The ADC would load the accelerometer too much and the voltage will drop; the buffer will prevent that.

Digital buffers are used as logic inverters (inverting buffers) or as simple buffers similar to the analog example above (to increase fan-out capabilities or to protect circuits against shorts when going to an external connector).

 

[koolguy]

Hi, in digital circuits the buffers used are also inverting and non inverting??