voltage divider bias combination

[atferrari]

When I look back up this post, I can see it now. The gain is calculated by dividing collector at voltage by voltage at emitter. When emitter voltage increases then the ratio of collector voltage to emitter voltage will reduce and so the gain reduces. That's something I learned on this post. Sorry, should have seen that one.

Well, for me, the explanation here below is the most clear I ever read. Short and to the point. Now I understand!

Anyway, the negative feedback is from collector to emitter, and essentially works like this:

The collector current is 'near enough' the same as the emitter current (ignoring the extra tiny base current in the emitter).

So if the collector resistor is 2000 ohms, the emitter resistor is 1000 ohms, and the current is 1mA, the voltage drop across the emitter resistor is 1V and across the collector resistor is 2V - so a gain of 2. To calculate it - 2000/1000=2. Simple ohms law.

If you drop the emitter resistor to 100 ohms, with the same 1mA, you now have only 0.1V across the emitter resistor, so a gain of 20 (calculated as 2000/100=20).

Make it 10 ohm, a gain of 200 (2000/10=200).

This is all DC and AC gain, but by placing a capacitor across the emitter resistor you can increase the AC gain (as you're lowering the AC emitter 'resistance'), while keeping the DC gain the same.

So back to the first example, 2000 and 1000 ohms, gain of 2 - now put a capacitor across the emitter, this essentially 'shorts' the emitter resistor out for AC, giving a gain of 2000/0=infinity. Obviously this doesn't happen, as it's not zero ohms, and the transistor only has a certain amount of gain - but by doing so you get the maximum gain possible from the transistor, as there's zero negative feedback.

On the 'bad side', zero negative feedback only means the lowest quality possible, and the stage isn't terribly linear - but it's all swings and roundabouts.

By applying 'reasonable' amounts of negative feedback you get better quality, and a MUCH more reproducible circuit - which is why it's commonly done that way.

 

[JimB]

Lets say we have a transmitter with 10 Watts output, and we need 100 Watts out.

What I am very greatly interested in is how to amplify microwatts or nanowatts of incoming RF in my crystal radio.

The reason I used the example of an RF power amplifier, was to highlight the importance and significance of the expression "linear" when applied to amplifiers. This seemed to be causing you some confusion.

What I am very greatly interested in is how to amplify microwatts or nanowatts of incoming RF in my crystal radio.

The circuit is drawn on the first posting way above but can be seen here.

The best suggestion that I can make is that you select a better circuit than the one which you show above.
The sensitivity of that circuit will be quite low, and will need a bit more that three feet of wire for an antenna.
It will also need an earth connection, which is implied by the triangle symbol, but not obviously spoken about.

The antenna, earth and amplifier input are connected directly across the tuned circuit.
This will lead to damping of the circuit resulting in poor selectivity.

The control marked "Regeneration" will give variable gain to the amplifier, but I don't see how it can be considered a regeneration control in any way.
Regeneration usually involves positive feedback, almost to the point of oscillation, resulting in increased gain in the amplifier. I see no evidence of such a mechanism in this circuit.

JimB

 

[audioguru]

I remember 60 years ago the AM radio I built had a 365pF tuning capacitor but this one is only 80pF so it must be a "shortwave" RF amplified crystal radio.
My radio had AGC for good sensitivity and freedom from overload and had 5 tuned LC circuits for good selectivity.

Since this one does not have Automatic-Gain-Control like all modern radios then weak signals will not be heard and strong signals will be overloading to severe distortion.
It has only one tuned LC circuit so its poor selectivity might pickup a few strong signals at the same time.

 

[teachlit]

The reason I used the example of an RF power amplifier, was to highlight the importance and significance of the expression "linear" when applied to amplifiers. This seemed to be causing you some confusion.

Confusion is very much the right word for my state of mind.
I expect words like 'feedback' to mean something is given or moves from one place to another but find people using it to mean that a value reduces. In the circuit I showed at the beginning, I depicted a 500 ohm resistor. With this resistor in place, the 'gain' or ratio between the voltage across the collector resistor and the voltage across the emitter resistor came to a certain value. If this resistor value of 500 ohm is reduced, then the gain or ratio is increased. But this reduction is described as 'negative feedback'. Yes, I am confused. I want to describe it as 'gain increase', not negative feedback. But, on reflection, I am learning to speak in electronics so I have to pay attention to the words that are used. Each time I see that a resistor value reduces, I can substitute 'negative feedback' and hope to get it right. What could be more confusing?
With the word 'linear' I feel on more solid ground. An AC signal enters an amplifier, the amplifier sends out the same signal at the other end but the separate points of the voltage trace are all multiplied by the same amount as if a magnifying glass were applied to a graph of the signal and a larger version of the graph appears with no clipping of the top or bottom parts of the signal.

 

[Nigel Goodwin]

Confusion is very much the right word for my state of mind.
I expect words like 'feedback' to mean something is given or moves from one place to another but find people using it to mean that a value reduces. In the circuit I showed at the beginning, I depicted a 500 ohm resistor. With this resistor in place, the 'gain' or ratio between the voltage across the collector resistor and the voltage across the emitter resistor came to a certain value. If this resistor value of 500 ohm is reduced, then the gain or ratio is increased. But this reduction is described as 'negative feedback'. Yes, I am confused. I want to describe it as 'gain increase', not negative feedback. But, on reflection, I am learning to speak in electronics so I have to pay attention to the words that are used. Each time I see that a resistor value reduces, I can substitute 'negative feedback' and hope to get it right. What could be more confusing?
With the word 'linear' I feel on more solid ground. An AC signal enters an amplifier, the amplifier sends out the same signal at the other end but the separate points of the voltage trace are all multiplied by the same amount as if a magnifying glass were applied to a graph of the signal and a larger version of the graph appears with no clipping of the top or bottom parts of the signal.

I think you're confusing yourself?.

'Feedback' (either negative or positive) is something 'fed back' from an output to an input. It's much easier to understand using opamps, as you have both positive and negative inputs - so you feedback to the negative input for negative feedback, and to the positive input for positive feedback.

It's not as clear cut in transistor circuits - in this particular case the negative feedback is from the collector to the emitter, with the transistor been used in 'common base' for this action.

 

[teachlit]

I think you're confusing yourself?.

It's not clear cut to me.

in this particular case the negative feedback is from the collector to the emitter, with the transistor been used in 'common base' for this action.

This sentence needs translation for me. What is moved from collector to emitter? Is it current? The emitter resistor reduces in value and something moves from collector to emitter. I can see that the gain ratio increases. So how can a gain increase be described as negative? Sorry Nigel. You are doing your best to explain and I am struggling to understand. I am near the point of going QRT (shut down the station in ham speak) but really want to understand this knowledge.

 

[Nigel Goodwin]

It's not clear cut to me.

This sentence needs translation for me. What is moved from collector to emitter? Is it current? The emitter resistor reduces in value and something moves from collector to emitter. I can see that the gain ratio increases. So how can a gain increase be described as negative? Sorry Nigel. You are doing your best to explain and I am struggling to understand. I am near the point of going QRT (shut down the station in ham speak) but really want to understand this knowledge.

A gain 'increase' isn't negative - it's achieved by applying LESS negative feedback. If you don't have any negative feedback to start with, you can't lessen it, so you can't increase gain in that way.

Negative feedback can only decrease gain. Feedback is basically a 'signal' it could be AC, DC, or both.

For a VERY crude simile think of a bucket under a running tap.

With the water flow constant it will take a certain time for it to fill up.

If you punch a hole in the bottom of the bucket, the bucket will take longer to fill up (small amount of negative feedback).

If you now punch a bigger hole in the bucket, it will take still longer (more negative feedback).

But no matter how big you make the hole (even more negative feedback) you can't make the bucket fill any faster than no hole at all (zero negative feedback), you can only make it fill slower (lower gain).

 

[teachlit]

A gain 'increase' isn't negative - it's achieved by applying LESS negative feedback. If you don't have any negative feedback to start with, you can't lessen it, so you can't increase gain in that way.

Negative feedback can only decrease gain. Feedback is basically a 'signal' it could be AC, DC, or both.

For a VERY crude simile think of a bucket under a running tap.

With the water flow constant it will take a certain time for it to fill up.

If you punch a hole in the bottom of the bucket, the bucket will take longer to fill up (small amount of negative feedback).

If you now punch a bigger hole in the bucket, it will take still longer (more negative feedback).

But no matter how big you make the hole (even more negative feedback) you can't make the bucket fill any faster than no hole at all (zero negative feedback), you can only make it fill slower (lower gain).

Thank you. This does help to drive away some of the confusion. The increase in gain is achieved by reducing the amount of negative feedback. I appreciate you taking the time to describe things in a more explicit way for me.
This leads me to ask ' what would be a way to provide positive feedback in this circuit?