# TRANSISTOR AC Amplification

#### greenberet

Joined Nov 13, 2006
9
When we bias a transistor in the active region using a single supply, and take the output across Vce , the ac waveform is riding on some DC, and so we use the coupling capacitor at the output.

In this site, i read in the section of the op-amp that the opamp uses a dual power supply (ie +15 and -15) so that the resulting output is pure AC, without any DC component. But i am not understanding how this is achieved.

Can you show how me this sort of method can be used to get a PURE ac in a BJT transistor amplifier, common emitter circuit, without the help of coupling capacitors..

#### Papabravo

Joined Feb 24, 2006
14,863
Sure! Use a bipolar supply. Collector goes to +15 through load resistor. Emitter goes to -15 through an R and a C. Base is biased at Ground.

BTW even opamps have small DC components because you never get zero output for zero input. The parameter that describes this is called Vos, that's V-sub-o-s, and it gives the value of the input offset voltage that produces a zero output.

#### Ron H

Joined Apr 14, 2005
7,014
Sure! Use a bipolar supply. Collector goes to +15 through load resistor. Emitter goes to -15 through an R and a C. Base is biased at Ground.

BTW even opamps have small DC components because you never get zero output for zero input. The parameter that describes this is called Vos, that's V-sub-o-s, and it gives the value of the input offset voltage that produces a zero output.

That still won't allow you to have zero volts on the collector, because you would then have Vcb=0, which is not an ideal operating point for a linear amplifier.

#### greenberet

Joined Nov 13, 2006
9
thats what i am saying. I tried the +15 and -15 bias but it still wont give Vcb zero. Then I wonder WHY the dual supply is given to the opamp.

#### Papabravo

Joined Feb 24, 2006
14,863
... you would then have Vcb=0, which is not an ideal operating point for a linear amplifier.
Never claimed it was ideal just closer to his requirements. I have no opinion on the requirements.

#### Ron H

Joined Apr 14, 2005
7,014
thats what i am saying. I tried the +15 and -15 bias but it still wont give Vcb zero. Then I wonder WHY the dual supply is given to the opamp.
Keep in mind that an op amp is not a common emitter amplifier. Look at the equivalent circuits in a few op amp datasheets, and also keep in mind that these are generally simplified.

#### greenberet

Joined Nov 13, 2006
9
Yea but in this very site it is written,

If it is necessary for an amplifier to be able to output true AC voltage (reversing polarity) to the load, a split DC power supply may be used, whereby the ground point is electrically "centered" between +V and -V. Sometimes the split power supply configuration is referred to as a dual power supply.
The amplifier is still being supplied with 30 volts overall, but with the split voltage DC power supply, the output voltage across the load resistor can now swing from a theoretical maximum of +15 volts to -15 volts, instead of +30 volts to 0 volts. This is an easy way to get true alternating current (AC) output from an amplifier without resorting to capacitive or inductive (transformer) coupling on the output. The peak-to-peak amplitude of this amplifier's output between cutoff and saturation remains unchanged.

And the opamp has a unbalanced output Diff amp in its second stage, and in textbooks it is written that the third stage is a level shifter. If o/p is already at ground then why use a level shifter, and if a level shifter exists then why use a dual power supply.

#### Ron H

Joined Apr 14, 2005
7,014
Yea but in this very site it is written,

If it is necessary for an amplifier to be able to output true AC voltage (reversing polarity) to the load, a split DC power supply may be used, whereby the ground point is electrically "centered" between +V and -V. Sometimes the split power supply configuration is referred to as a dual power supply.
The amplifier is still being supplied with 30 volts overall, but with the split voltage DC power supply, the output voltage across the load resistor can now swing from a theoretical maximum of +15 volts to -15 volts, instead of +30 volts to 0 volts. This is an easy way to get true alternating current (AC) output from an amplifier without resorting to capacitive or inductive (transformer) coupling on the output. The peak-to-peak amplitude of this amplifier's output between cutoff and saturation remains unchanged.

And the opamp has a unbalanced output Diff amp in its second stage, and in textbooks it is written that the third stage is a level shifter. If o/p is already at ground then why use a level shifter, and if a level shifter exists then why use a dual power supply.
Op amp internal configurations vary, but voltage mode op amps always have a balanced differential input stage. The output of that stage will always have a common mode level which is not at ground (assuming the inputs are at ground). The second stage may or may not be another diff amp, but there is always one or more gain stages following the input stage, and then usually a voltage follower to provide low output impedance. The interior gain stage(s) provide level shifting.
Why use dual supplies? If you only have a single positive supply, how do you expect the output voltage of any circuit to go below ground?

#### greenberet

Joined Nov 13, 2006
9
But I am finding it hard to believe.

Can you give me an example of a circuit with dual power supply where o/p varies between + and - polarities.

#### Ron H

Joined Apr 14, 2005
7,014
But I am finding it hard to believe.

Can you give me an example of a circuit with dual power supply where o/p varies between + and - polarities.
Sure. Here's a 3-transistor "op amp" configured for gain of +2.
Is this a homework assignment?

#### Attachments

• 13.2 KB Views: 58
• 28.7 KB Views: 51