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Common Emitter Amplifier - Class B

Chr1s

Dec 20, 2016
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Hi All,

I am designing a Class-B amplfier in tina using a BC109 transistor with a calculated gain of 5. I have attached a screenshot of the the AC transfer characteristic, however i dont really understand what it means? Can someone help me to understand what is going on?
 

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bertus

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Hello,

Do you have a complete schematic?
The behaviour could come from the capacitors used in the circuit.

Bertus
 

Harald Kapp

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How about a schematic diagram?
Our crystal balls have collectively gone on holidays;)
 

Chr1s

Dec 20, 2016
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Sorry Guys. Attached is the circuit in Tina 9
 

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Audioguru

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Without base bias current, the transistor is turned off and does not conduct unless the input voltage is fairly high. Look at the current, it is very very close to zero. With a fairly high input voltage then a signal at the output would have severe distortion.
Also, if it is biased then it would be class-A, not class-B. Look in Google for class-B that uses two transistors and both have no bias.
 

BobK

Jan 5, 2010
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A class B amplifier requires 2 transistors, 1 for the positive half and one for the negative half of the signal. One a PNP and the other an NPN.

Bob
 

Ylli

Jun 19, 2018
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Class B is defined as an operating point such that the active device conducts over 180° of the input signal. If you take your circuit and add a bias network that holds the bias at a point where the transistor is *just* starting to conduct, you will have a class B amplifier. In the single ended configuration, it is not very linear and the output looks more like a half wave rectifier than the input.
Annotation 2020-02-10 194415.png
 

BobK

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Not much of an amplifier if it ignores half of the signal. Maybe it's a class B half-amplifier.

Bob
 

WHONOES

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If the amp is biased to have a dc current flowing through it, it is class A.
 

Ylli

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If the amp is biased to have a dc current flowing through it, it is class A.
And if it is biased so that there is no current flowing, then it is likely in class C. Class B means 180° conduction angle. Impossible to maintain in real life. What I posted has 3 uA of idle current. About as close to Class B as you are going to get.
 

Ylli

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Class A is defined as the active device conducting over 360° of the input waveform. Here is the sim showing the input voltage and the transistor Ic.
Annotation 2020-02-12 123330.png Annotation 2020-02-12 124117.png
Are you really going to tell me you see the transistor conducting over the full 360° of the input waveform?
 

Harald Kapp

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@Ylli : Understanding these waveforms would be greatly simplified if you had labeled the nodes with "speaking" names (F4 key in LTSpice) or at least indicated which node is n002 :confused:.

Are you really going to tell me you see the transistor conducting over the full 360° of the input waveform?
Having the transistor not correctly biased and thus not being conductive over the full 180 ° doesn't mean this is not a class A amplifier.
To be classified as class B the second transistor is missing which would fill in during the non-conducting phase of Q1.
 

Ylli

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Understanding these waveforms would be greatly simplified if you had labeled the nodes with "speaking" names (F4 key in LTSpice) or at least indicated which node is n002
point taken
not being conductive over the full 180° [360°] doesn't mean this is not a class A amplifier.
Yes it does. By definition a class A amplifier conducts over the full 360°. If it does not conduct over the full 360° it is not class A.
Having the transistor not correctly biased
The device *is* correctly biased for operation as a class B amplifier.


There are no requirement under the definition of 'Class B' that more than one active device needs to be used. From Wiki:

"In a class-B amplifier, the active device conducts for 180 degrees of the cycle. This would cause intolerable distortion if there were only one device, so two devices are usually used, especially at audio frequencies. Each conducts for one half (180°) of the signal cycle, and the device currents are combined so that the load current is continuous.

At radio frequency, if the coupling to the load is via a tuned circuit, a single device operating in class B can be used because the stored energy in the tuned circuit supplies the "missing" half of the waveform. Devices operating in Class B are used in linear amplifiers, so called because the radio frequency output power is proportional to the square of the input excitation voltage. This characteristic prevents distortion of amplitude-modulated or frequency-modulated signals passing through the amplifier. Such amplifiers have an efficiency around 60%."

Now go back and look at my reply's #9 and #12. A single ended class B amp is not useful at audio frequencies, but as an RF amplifier, with a tuned network as the load, it works just fine. The posted circuit is properly biased and operating as a class B RF amplifier.
 
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BobK

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"In a class-B amplifier, the active device conducts for 180 degrees of the cycle.
That definition is not very useful. Consider a symmetric square wave with 90% of the cycle positive and 10% negative. Would a class B amplifier conduct over half the cycle?

Bob
 

Ylli

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That definition is not very useful.
Useful to you or not, that *is* the definition.
Consider a symmetric square wave with 90% of the cycle positive and 10% negative.
Apples and oranges. The amplifier classes are defined with sine wave inputs. For what you describe, you are not using an amplifier, you are using a switch.
 
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