Understanding the Basics of Voltage Amplifiers

[salsero]

Could you please tell me what the inputs and the outputs of a VOLTAGE AMPLIFIER are?

Also what do these mean (in a voltage amplifier):
- voltage gain
- bandwidth
- distortion
- positive/negative feedback
?

 

[chroot]

Originally posted by salsero
Could you please tell me what the inputs and the outputs of a VOLTAGE AMPLIFIER are?

The input is a voltage. The output is another voltage, related in some way to the input. Usually, the output voltage is larger. For example, a cassette tape player reads the magnetic field on a tape, and produces a very small-amplitude voltage signal. A voltage amplifier increases that voltage many times so that speakers can be driven to produce sound.

Also what do these mean (in a voltage amplifier):
- voltage gain

Gain refers to the ratio between output and input. If the amplifier makes the outgoing signal twice the amplitude of the incoming signal, its gain is two. When you turn up your radio, you're increasing the gain of an amplifier.

- bandwidth

This refers to (generally) the highest frequency that can be amplified properly by the amplifier. The gain of an amplifier is really dependent upon the incoming frequency -- it doesn't do as well at amplifying high frequencies. The bandwidth is often specified at the so called "-3 dB point," which means nothing more than "half." Let's say an amplifier is specified as having a gain of 2 and a bandwith of 1 MHz. This means that at 1 MHz, the gain is half of its maximum -- i.e. its gain at 1 MHz is just one. At 2 MHz, the amplifier may only be producing 1/10 of its nominal gain.

- distortion

An ideal amplifier produces an exact copy of its input, just at a larger amplitude. Real amplifiers also change the signal somewhat, and make the output waveform a little different from the input. This change is called 'distortion.' When the amplifier in a radio produces a lot of distortion, the sounds coming out sound funny. Certain frequencies are enhanced, while others are attenuated.

- positive/negative feedback
?

An amplifier that samples its own output and adjusts itself to keep its output steady is a "feedback" amplifier. When an amplifier samples its output and uses that information to reduce its gain, it's called negative feedback. When the amplifier uses that information to increase its gain, it's called positive feedback.

Most of the amplifiers you're familiar with are negative feedback, since negative feedback leads to stability.

In some instances, you don't want stability. An example of a positive-feedback amplifier is a comparator. This is a device that determines when one signal is higher than another. When one signal is above the other, the output rapidly swings all the way to the supply voltage. When the signal is below the other, the output rapidly swings all the way down to ground. This action is due to positive feedback. As soon as the output is slightly high, it gets higher and higher until it hits the supply voltage. As soon as the output voltage is slightly low, it gets lower and lower until it hits ground. It "snowballs," and positive feedback results in such behavior.

Does this make sense?

- Warren

 

[salsero]

Great answer, Warren! Thank you! Now I know much more about the voltage amplifier. But somehow it still isn't enough. I need to find an answer to the following question:

In a voltage amplifier, which of the following is NOT usually a result of introducing negative feedback?
(A) Increased amplification
(B) Increased bandwidth
(C) Increased stability
(D) Decreased distortion
(E) Decreased voltage gain

 

[chroot]

Originally posted by salsero
In a voltage amplifier, which of the following is NOT usually a result of introducing negative feedback?
(A) Increased amplification
(B) Increased bandwidth
(C) Increased stability
(D) Decreased distortion
(E) Decreased voltage gain

Well, now it sounds like you just want me to do your homework for you... which isn't acceptable here.

This question is very easy -- the answer is actually in my last response.

- Warren

 

[salsero]

This is not homework. This is a question from the sample Physics GRE exam (question 72), and the correct answer is (A).

When I saw this question I understood that it would be better if I knew something about voltage amplifiers before taking the exam. This is why I asked these questions.

Your answers were very informative, but I still couldn't understand from them how a negative feedback would affect bandwidth and distortion, and why would there be a decrease in voltage gain.

 

[enigma]

Originally posted by chroot

An amplifier that samples its own output and adjusts itself to keep its output steady is a "feedback" amplifier. When an amplifier samples its output and uses that information to reduce its gain, it's called negative feedback. When the amplifier uses that information to increase its gain, it's called positive feedback.

Warren hit it pretty much on the head.

If you're using negative feedback, it's because the amplifier will switch too quickly from 0 gain to +HI gain with only a very small increment to the input voltage if it was by itself.

IIRC (correct me if I'm misremembering, please), a negative feedback loop consists of a resistor connected to both sides of the amplifier. With that in place, increasing the input voltage causes some current to filter back through the resistor, decreasing the input voltage, and making the output voltage able to produce a smaller increment.

 

[chroot]

Originally posted by salsero
Your answers were very informative, but I still couldn't understand from them how a negative feedback would affect bandwidth and distortion, and why would there be a decrease in voltage gain.

All you really need to know is that A is NOT a result of negative feedback -- obviously negative feedback will not result in larger gain.

The other effects of negative feedback are not easy to prove, or understand!

- Warren

 

[chroot]

Originally posted by enigma
IIRC (correct me if I'm misremembering, please), a negative feedback loop consists of a resistor connected to both sides of the amplifier. With that in place, increasing the input voltage causes some current to filter back through the resistor, decreasing the input voltage, and making the output voltage able to produce a smaller increment.

That would be a current-feedback amplifier (which is the most common kind). There are also amplifiers available that have voltage-feedback topologies.

- Warren

 

[flexifirm]

Food for thought

There really is not such thing as a VOLTAGE AMPLIFIER!

It's usually a small signal POWER AMPLIFIER or ATTENUATOR or POWER TRANSFORMER (whichever way you look at it).


We just look at is as a voltage amplifier to simplify calculations, etc. But there is no such thing as voltage energy alone- energy is composed of voltage and current.

Therefore there is no such thing as a voltage amplifier, as it is in fact a power amplifier where the currents are neglected or overlooked or insignificant for calculations.