Supply Voltage Design criteria in Amplifiers

[aurosunil]

I am trying to design an amplifier using 2N5088 bipolar transistor. I am using voltage divider biasing.
I want to know what value of Vcc I should use and why? I have gone through the data sheet but could not find any help in this direction. This is my first attempt on the application side. Can any one help me?

sunil :?

 

[Nigel Goodwin]

I'm not very sure what you are asking?, the supply voltage isn't particularly dependent on the transistor you use, but on other factors - mainly the voltage swing you need. The main concern for the transistor is that it can handle the voltage you provide it.

 

[chemelec]

The Second main concern is adjusting the Base Bias Correctly.

In a typical class A amplifier, with no signal present, the collector voltage should be 1/2 Supply, Plus .6 volts.

So for a 12 volt supply, the collector voltage would be 6.6 volts.
This should give the Best Symetrical Output Signal.

The Supply Voltage should Not Exceed 30 Volts for this transistor.
I Also like and use the 2N5088.

 

[aurosunil]

...

The Supply Voltage should Not Exceed 30 Volts for this transistor.
I Also like and use the 2N5088.

Thank you Chemelec for response.

What are the factors on whic this supply voltage depends? On what basis you said that Supply Voltage for this transistor or any other should not exceed 30 Volts?

 

[audioguru]

what basis you said that Supply Voltage for this transistor should not exceed 30 Volts?

It is the very first absolute maximum rating on its datasheet.

 

[chemelec]

Your Supply voltage is determined by the Output Voltage Swing that you need. And I Don't really Recommend 30 Volts.

The Output voltage swing will Typically be about 1 volt less than the supply voltage, but this also depends on your collector resistor.

The Current will partly be a Function of the Collector Resistor and Emitter resistor.

If you are going for Voltage Gain, I would recommend a 2K2 to 10K on the Collector.

An Emitter resistor will help with bias as well as protect the transistor from excessive current. (Possibly a 470 to 1K resistor)
Bypassing this resistor with a Capacitor, will help with Low Frequency Response. (.1uF to 100uF)

This Info is Very General.
If you need more info, I would suggest you do some Research, or a lot of expermenting.

 

[aurosunil]

What i understand is following in general:
1. Supply Voltage Vcc can not be more than maximum value of Vce.
2. Vcc is limited by the output voltage swing.
3. Once Vcc is decided Ib should be chosen for Q point
4. Then other resistors must be finalised.
Am i correct?

 

[audioguru]

What i understand is following in general:
1. Supply Voltage Vcc can not be more than maximum value of Vce.
2. Vcc is limited by the output voltage swing.

Usually the voltage swing is selected first, Vcc is matched to it and a transistor is selected with a high enough Vce.

3. Once Vcc is decided Ib should be chosen for Q point

Since a bare transistor produces a lot of distortion and its Q-point isn't stable with temperature changes, negative feedback is used to assist with biasing the transistor, reducing distortion and making it more temperature stable. :lol:

 

[aurosunil]

This is how i started stting the dc biasing:
I choose 15V as Vcc. [since Vceo = 30V taking a factor of safety=2].
and taking Vc = 0.5Vcc = 7.5V

Now 2N5088 datasheet On characterstics says that
min hfe = 300 at Ic=100microA and Vce = 5.0V
So taking Ic = Icq = 100microA, Rc=(Vcc-Vc)/Icq = 75K

Ve =Vc - Vce = 7.5-5.0 = 2.5V
as Ie = Ic
Re = Ve/Ic = 25K

Vb = Ve+0.7 =3.2V and calculating the Voltage divider biasing resistors which came out to be R1=2.5M and R2=680K

Although i have done all this i am not sure if it is alright. In the datasheet i do not find Collector characterstic curves i.e. Ic Vs Vce so that i can draw the load line and find the Q point values. and i feel all is not well.
Can some one help me out?

 

[audioguru]

It looks pretty good but its max output swing won't be symmetrical because the emitter voltage is about 2.3V. If the emitter resistor is bypassed for max gain then the collector voltage should be half-way between 2.4V and 15V.

It has a high 75k output resistance so must feed a load of 750k or more to keep its gain from dropping due to loading.

 

[aurosunil]

It is all right but I am still puzzled about what is the criteria for the following:

1. How to select Ve :?: Is there any principle? :?:
2. In almost all the books it is written that dc load line should be drawn to fix the Q-point. But in the datasheets I don't find collector charcterstics so how to draw the dc loadline :?:
:?

 

[audioguru]

It is all right but I am still puzzled about what is the criteria for the following:

1. How to select Ve :?: Is there any principle? :?:
2. In almost all the books it is written that dc load line should be drawn to fix the Q-point. But in the datasheets I don't find collector charcterstics so how to draw the dc loadline :?:
:?

I haven't drawn a load line for about 40 years. I used them with very nonlinear germanium transistors. Your text books were probably made before linear silicon transistors and negative feedback were invented. My 1968 Philips Transistors Data Handbook has collector curves.

I select RE so that the emitter voltage is raised at least 1V. Then temperature won't affect the transistor much. I select RE in a ratio with RC//load for low gains.

I calculate a voltage for the Q-point halfway between when the transistor is cutoff and when it is saturated. That's all. :lol:

 

[aurosunil]

I haven't drawn a load line for about 40 years. I used them with very nonlinear germanium transistors. Your text books were probably made before linear silicon transistors and negative feedback were invented. My 1968 Philips Transistors Data Handbook has collector curves.

I select RE so that the emitter voltage is raised at least 1V. Then temperature won't affect the transistor much. I select RE in a ratio with RC//load for low gains.

I calculate a voltage for the Q-point halfway between when the transistor is cutoff and when it is saturated. That's all. :lol:

Thank you audio for giving me this tip it has really helped me just one more question please:
Transistors datasheets gives ON Charcterstics i.e. there are listed hfe values for various values of Ice and Vce. It means that dc biasing should be done for the particular value of Ic corresponding to that hfe value upon which we base our design :?
say for example 2N5088 Ic = 100micro amp if we design for hfe = 300 :?:

 

[audioguru]

Transistors datasheets gives ON Charcterstics i.e. there are listed hfe values for various values of Ice and Vce. It means that dc biasing should be done for the particular value of Ic corresponding to that hfe value upon which we base our design :?
say for example 2N5088 Ic = 100micro amp if we design for hfe = 300 :?:

The 2N5088 could have an hFE anywhere from 300 to 900 at 100uA. Therefore it varies so much that you should never design a transistor circuit for a certain hFE such as 300, or it might not work with transistors with an hFE of 800 or 900.
Just have a current in the voltage divider for its base bias of at least 10 times the transistor's base current at its lowest hFE. Then the voltage divider's voltage isn't changed by the base current and the circuit will work with any 2N5088 transistor. :lol:

 

[aurosunil]

The 2N5088 could have an hFE anywhere from 300 to 900 at 100uA. Therefore it varies so much that you should never design a transistor circuit for a certain hFE such as 300, or it might not work with transistors with an hFE of 800 or 900.
Just have a current in the voltage divider for its base bias of at least 10 times the transistor's base current at its lowest hFE. Then the voltage divider's voltage isn't changed by the base current and the circuit will work with any 2N5088 transistor. :lol:

But how to calculate this base current :?: I do not find any mention of base current in the datasheets! Only Ic is given at a given hFE in the On Characterstics.
:idea: Is it that we take Ic for the lowest hFE say 100micro amp in the case of 2N5088 divide it by 300 (lowest hFE) to get the Base current and multiply the result by 10 to get the acutal base current to be set up using voltage divider circuit! am i right :?: i.e. 3.3 micro amp in this case.

 

[audioguru]

Correct. Base current is collector current divided by hFE.
But you shouldn't call the divider's current a "base current". The base current will be 1/10th or less the divider's current.

If you want the transistor to saturate well, you should allow more base current since hFE drops off at low Vce voltages. :lol:

 

[aurosunil]

Correct. Base current is collector current divided by hFE.
But you shouldn't call the divider's current a "base current". The base current will be 1/10th or less the divider's current.

If you want the transistor to saturate well, you should allow more base current since hFE drops off at low Vce voltages. :lol:

Thank you audio for the patience. I hope everything is clear to my mind at this juncture. I will do some practical work and come back again if need be there.
Thanks once again