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Biasing resistors

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zachtheterrible

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Just a quick question: Say we have an NPN common emitter amplfier. What is the point of having biasing resistors from the base to ground and to Vcc? We bias an NPN transistor negatively, so what is the point of having a resistor from base to Vcc?
 
zachtheterrible said:
Just a quick question: Say we have an NPN common emitter amplfier. What is the point of having biasing resistors from the base to ground and to Vcc? We bias an NPN transistor negatively, so what is the point of having a resistor from base to Vcc?

Actually you bias an NPN transistor from the posititive rail!.

The reason for using a potential divider to bias the transistor is to swamp variations due to the gain spread of different transistors. You can just have a bias resistor from base to Vcc, but you have to select it's value for every single one you build. Another option is to place a bias resistor from base to collector, and the negative feedback sets the DC conditions and compensates for the differing transistor gains - however, the negative feedback also lowers your gain.

The classic circuit is four resistors, one in the collector, one in the emitter, and a potential divider on the base - this makes it as simple as possible to design.
 
Say we have an NPN common emitter amplfier. What is the point of having biasing resistors from the base to ground and to Vcc? We bias an NPN transistor negatively, so what is the point of having a resistor from base to Vcc?
I don't know if there is a benefit of connecting a resistor from base to ground.

BUT, a voltage divider allows you to define a specific current and a specific voltage.
 
zachtheterrible said:
Just a quick question: Say we have an NPN common emitter amplfier. What is the point of having biasing resistors from the base to ground and to Vcc? We bias an NPN transistor negatively, so what is the point of having a resistor from base to Vcc?
Zach, Nigel's description is very good. Basically the procedure is to set the base voltage with the voltage dividing resistors. Say we make it 1.6 Volt for example.

There will be about 0.6 V across the B-E junction so the emitter voltage will be about 1 V. If you had say a 1k emitter resistor to gnd, then the emitter current will be about 1V/1k = 1 mA. Most of the emitter current flows into the collector so the collector current is near enough to 1 mA also.
 
To add to what Nigel said, using two resistors in a voltage divider at the base not only stabilizes bias from one transistor to the next, but it also stabilizes bias over temperature too which is extremely useful.
 
When I said resistor from base to ground I was thinking of having the transistor turn on at a certain point. I got to thinking about that later but it was too late, I was away from the computer :lol:

So its basically just to stabilize from one transistor to the next then? and temperature as ron added. I can see how variations in the transistor's gain would affect the circuit, but I don't see how placing the resistor from base to ground will keep the transistor's variation from changing the circuit too much.
 
zachtheterrible said:
So its basically just to stabilize from one transistor to the next then? and temperature as ron added. I can see how variations in the transistor's gain would affect the circuit, but I don't see how placing the resistor from base to ground will keep the transistor's variation from changing the circuit too much.

Draw the circuit and do the maths!, it's really VERY simple.

1) The base voltage is simply calculated from the potential divider values.

2) The emitter voltage is 0.7V lower than the base voltage.

3) The emitter current can be calculated from the emitter voltage and the value of the emitter resistor.

4) The collector current is the emitter current MINUS the base current, but the base current is only very small and can generally be ignored. So for this simple example, collector current equals emitter current.

5) The collector voltage can be calculated from the collector current and the collector resistor.

Notice that the transistors gain appears nowhere (I ignored it in part 4).

If you would now like to try introducing it?, the emitter current is the sum of base current and collector current - try doing the sums for different gain values?. Notice that if the gain of the transistor is only 100, the base current is only 1% of the emitter current, and for a gain of 200 only 0.5% of the emitter current - these sorts of figures can safely be ignored if you are using 5% resistors!.
 
One thing i always forget when i couple a signal to a potential divider biased transistor amplifer, is that the capacitor and the resistor to ground form a high pass passive filter, incase you're using low freq signals.

Megamox
 
Megamox said:
One thing i always forget when i couple a signal to a potential divider biased transistor amplifer, is that the capacitor and the resistor to ground form a high pass passive filter, incase you're using low freq signals.

You've also forgot a couple of other things as well!.

1) It's BOTH resistors in parallel that you need to consider (the two halves of the potential divider are effectively in parallel as far as AC is concerned).

2) Also in parallel is the input impedance of the transistor - not so simple to calculate as the two resistors!. The design values of the potential divider should swamp that fairly well though.
 
The input impedance of a common-emitter transistor stage with its emitter AC-grounded could be as low as 100 ohms if its collector current is about 60mA. It is less than 2k ohms at a few mA.
 

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Megamox said:
Yes good point, how about the reactance of the coupling cap, that would be included too i think.

The reactance of the coupling capacitor is the other leg of the high-pass filter, and should be calculated to find the low frequency drop-off point.
 
audioguru said:
The input impedance of a common-emitter transistor stage with its emitter AC-grounded could be as low as 100 ohms if its collector current is about 60mA. It is less than 2k ohms at a few mA.
1.What is the role of f=1KHz and Vce = 10V if it changes with the signal frequency then what should be the design frequncy for input impedence?
2. In books they say that input impedence can be taken as hFE times emitter resistance then why to use this graph :?:
3. In some books coupling capacitor is shown as a elctrolytic capacitor how it can be :?:
 
aurosunil said:
audioguru said:
The input impedance of a common-emitter transistor stage with its emitter AC-grounded could be as low as 100 ohms if its collector current is about 60mA. It is less than 2k ohms at a few mA.
1.What is the role of f=1KHz and Vce = 10V if it changes with the signal frequency then what should be the design frequncy for input impedence?

The 1KHz is simply the frequency that graph was plotted at, it doesn't say it's frequncy dependent, although it will be to some extent. 1KHz is just the standard frequency for audio measurements.

2. In books they say that input impedence can be taken as hFE times emitter resistance then why to use this graph :?:

Audioguru clearly pointed out the graph was for a transistor with a decoupled emitter resistor "common-emitter transistor stage with its emitter AC-grounded". So that formula doesn't apply.

3. In some books coupling capacitor is shown as a elctrolytic capacitor how it can be :?:

It mostly will be, due to the fairly low impedances involved, what's the problem with that?.
 
Electrolytic and ceramic capacitors cause some distortion at low frequencies when used as coupling caps due to their dielectric absorption.
Also their value changes with the instantaneous voltage across them.
These problems with those types of capacitors causes even-harmonics distortion when the capacitor is operating as a simple high-pass filter. Metalized plastic film caps are best for coupling caps.

An example of dielectric absorption:
1) Charge an electrolytic cap.
2) Discharge it by shorting it for a couple of seconds.
3) Measure its voltage with a high impedance voltmeter. It will show a voltage even though you thought it was discharged. :lol:
 
Nigel Goodwin said:
aurosunil said:
3. In some books coupling capacitor is shown as a elctrolytic capacitor how it can be :?:

It mostly will be, due to the fairly low impedances involved, what's the problem with that?.

Thanks for clarifying the first 2 points but still as far as 3rd question is concerned can you help a bit more.
My understanding of capacitors is that electrolytic capacitors are not used for ac applications and capacitor may blast if it connected wrong way and so the question. but it seems for low impedences electrolytic capacitor may be used for ac as well. Is there any criteria for low impedence?
 
Electrolytic caps are usually polarized so must not have a reverse voltage across them so they don't blow-up!

To couple AC frequencies to a low impedance like a speaker, cheap amplifiers use an electrolytic capacitor. The capacitor always has a DC voltage across it so its voltage never reverses.

Even cheap two-way speakers use non-polarized electrolytic caps to keep the woof out of the tweeter and to keep the tweet out of the woofer.
 
aurosunil said:
Nigel Goodwin said:
aurosunil said:
3. In some books coupling capacitor is shown as a elctrolytic capacitor how it can be :?:

It mostly will be, due to the fairly low impedances involved, what's the problem with that?.

Thanks for clarifying the first 2 points but still as far as 3rd question is concerned can you help a bit more.
My understanding of capacitors is that electrolytic capacitors are not used for ac applications and capacitor may blast if it connected wrong way and so the question. but it seems for low impedences electrolytic capacitor may be used for ac as well. Is there any criteria for low impedence?

Capacitors (of ANY kind) essentially pass AC and block DC, it doesn't matter if they are electrolytic or not. All you need to do for an electrolytic coupling capacitor (apart from ignore Audioguru's scare mongering 8) ) is ensure it has a DC potential across it the correct way.

Usually a coupling capacitor in a transistor circuit will connect from the collector of one transistor to the base of the next, both of these points will have a DC voltage on them - you just need to connect the positive of the electrolytic to the most positive of the two points.

There's also nothing particularly bad about speaker coupling capacitors, some of the most highly respected HiFi amplifiers in the world used them, and they still in great demand today!.
 
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