Attenuators and Amplifiers

[lord loh.]

In the block diagram of most instruments shown in my text books, I see an attenuator followed by an amplifier.

What is the need for this? Why not provide a lower gain amplifier without a preceding attenuator or a lower attenuation attenuator without a succeeding amplifier?

I have not found any satisfactory explanation for this anywhere.

Thanks in advance.

 

[Nigel Goodwin]

In the block diagram of most instruments shown in my text books, I see an attenuator followed by an amplifier.

What is the need for this? Why not provide a lower gain amplifier without a preceding attenuator or a lower attenuation attenuator without a succeeding amplifier?

I have not found any satisfactory explanation for this anywhere.

Assuming you mean meters and oscilloscopes?, it's quite simple, there's not much use having either with a single (low sensitivity) range. By amplifying the signal you increase the sensitivity, and to get higher ranges you use an attenuator in front of it. You could reduce the gain of the amplifier?, and some units do, but this only gives a limited range - so these units use attenuators as well! (notice the x10 gain switch on your scope, this INCREASES the gain by ten, but reduces bandwidth).

 

[lord loh.]

Not only oscilloscopes, I saw it on several instruments...

The wave analyser. It had a block diagram of

attenuator -> amplifier -> filters -> filter amplifier ->attenuator -> meter range amplifier.

Why not make the amplifier give gains lower than 1, if the input signals must really be reduced in amplitude? wont it serve the same purpose?

As per the block diagrams, both are operational at all times... So do I understand that if real attenuation is required, we reduce the amplifier gain and increase the attenuation? and vice versa for gain?

Thank you.

 

[Nigel Goodwin]

Not only oscilloscopes, I saw it on several instruments...

It's not only oscilloscopes, ANY instrument tend to do the same for exactly the same reasons.

 

[lord loh.]

I was just wondering what is the voltage gange needed to create a unit deflection on commercially available CRTs? is it in the order of mV? or uV?

Doesn't it depend on the accelrating potential?

This might help me understand a bit...

 

[Nigel Goodwin]

I was just wondering what is the voltage gange needed to create a unit deflection on commercially available CRTs? is it in the order of mV? or uV?

Doesn't it depend on the accelrating potential?

Yes it does, the higher the EHT used the faster the electron beam travels, and the harder it is to deflect.

A scope tube requires HUNDREDS OF VOLTS deflection, hence the amplifiers involved.

 

[lord loh.]

So when hundreds of volts are required for deflection, why have attenuators at all? most signals given to oscilloscopes is in the range of fe mV to 50V...

The highest signals of 50V shall also need amplification. :? :? :?

Are the attenuators in the signal path all the time? or are they brought only when needed? And is the same with amplifiers?

Thank you.

 

[Roff]

An amplifier that can amplify millivolt signals cannot handle 50 volt signals. The attenuators are there partly for that reason.

 

[lord loh.]

Okay I am begining to understand a bit now... But then how does one convert the mV into several hundred volts?

 

[Nigel Goodwin]

Okay I am begining to understand a bit now... But then how does one convert the mV into several hundred volts?

With an amplifier!.

The attenuator is basically a 'volume control', your stereo system at home is just the same, the amplifier runs at full gain all the time, and the volume control is an attenuator that reduces the signal going through it.

 

[lord loh.]

So the amplifiers are not adjustable at all????? :shock: :shock: :shock:

 

[Nigel Goodwin]

So the amplifiers are not adjustable at all????? :shock: :shock: :shock:

Not usually, there's no real reason for them to be! - although there are occasional exceptions (like the x10 switch on the front of a scope I mentioned earlier).

 

[lord loh.]

Okay... It mat be fine with the oscilloscope to have an attenuator followed by an amplifier but what about other instruments where hundreds of volts are not required?

A wave analyser block diagram I had in my text book, had the same case. The input was

Signal -> Attenuator -> Amplifier -> [Filters+Amp(active filter)] -> Attenuator -> Scale range amplifier -> meter.

Where meter was supposed to be a D'Arsovnal movement... Now that does not require hundreds of volts... just a few uA....

Now I guess that the third attenuator is meant to set ranges on the scale. Suppose we had a lower attenuation attenuator and no amplifier at all? What would suffer?

I am soory If I am gettin of people's nervers... But I still have not got 100% insight...

Thanks for all the help so far Nigel...

 

[Nigel Goodwin]

It's mostly to do with being versatile - so the unit can accept the widest possible range of signals.

An amplifier has what's called it's "dynamic range", which is the range between the smallest signal it can pass (before amplifier noise becomes a problem) and the highest it can pass (when the amplifier starts to clip). It's vital to keep within this range, and preferably within a smaller section of it, where performance will be the best.

It this particular example, you have a filter in the middle, and this will be even more signal level dependent than a plain amplifier. So the first amplifer is to get small signals amplified to a suiatable level to feed the filter - and it has an attenuator in front so it can take larger signals if required.

You can't generally feed a meter-movement directly, so there's an amplifier feeding that, to increase the sensitivity of the meter. The attenuator in front of that is to allow the meter a wider operating range, just as with your multimeter.

Having done a gig with my daughters band last night, perhaps I can use an audio example to try and make it more clear?.

You have an audio mixer for the band, with various vocal mikes and guitars etc. connected to it. EXACTLY like your diagram these first enter an attenuator, which sets the gain of the input (sometimes it actually alters the gain of the stage directly, by adjusting the feedback round an opamp). If you're direct injecting the guitars (or a keyboard), then the signal level is a LOT higher than a microphone, so you need a LOT more attenuation. There's then an amplifier stage that boosts the signal to a good level, well above the noise floor, but well before the onset of clipping. This signal is then fed through tone control stages (the filter stage in your instrument) - after that it goes to another attenuator control (the slider controlling the volume of that stage), it's then amplified again (just like in your instrument) and sent out to the power amplifier to drive the speakers (the meter-movement in your instrument).

This is all done to keep the signal at an optimum level as it passes through the mixer - just like in your instrument!.