AMPLIFIER CLASSES

If the title sounds like an invitation to go back to school you need to read the  which is right below this one.

 

As was explained previous post, an amplifier that is added on between a transmitter and the antenna is not necessarily a linear amplifier.  But there are a lot more amplifiers then just the ones added on after a transmitter to boost the power.   Amplifiers are the basic building blocks of analog electronics.

 

As stated the class of amplifier is determined by its level of bias.  The class A runs for 360º of every cycle.   That is to say when a cycle starts from zero then rises to the full plus voltage then drops back to zero followed by its negative swing back to zero is a full cycle and the class A amplifier is on with its output following through out the cycle.  The class A is the most linear but is the least efficient of the classes of amplifiers.

 

Efficiency is defined as the percentage of power output (AC/RF) to DC power input.  If you have an amplifier running on 100 volts with .1 amps DC then the amplifier is running with 10 watts input power.  If a wattmeter is placed on the output of that amplifier reads 2.5 watts then that amplifier has 25% efficiency which is about the maximum efficiency possible with a class A amplifier.

 

The efficiency can be increased by moving the bias up to cutoff and thus driving the amplifier to class B.  The class B amplifier runs for 180ºof every cycle.  The active amplifying device (Tube or Transistor) is on just half the time and off half the time during any cycle.  While the amplifier is linear as described in the last blog it will distort an audio signal because half of the cycle is cut off.  To compensate for this two tubes or transistors work together in such a way that while one device is off the other is on thus completing the cycle at the output.  This configuration is called push pull amplifiers.    The efficiency of the class B amplifier is raised to about 50%.  With no drive (input signal) the class B amplifier draws no current.

 

Because the active component is only operating 50% of the time it can run at higher power then it can in class A.  Though vacuum tube is ancient history it is easier, at least for this author, to use it to compare thus the 6C4 vacuum tube characteristics will be used for illustration. The reason for using this tube is because it was a popular tube for use for both audio and RF and the needed information was readily available from the RCA Receiving Tube Manual.  When run in class A configuration the maximum DC input, using the recommended 100 volts plate voltage, is 1.1 watts thus about .28 watts out.  When used for class B operation it could be run at about 2.2 watts DC input, if the plate voltage remained at 100 volts (Class B in vacuum tube operation can be operated at a higher voltage which allowed it to be operated at an even higher power range.)  The power output in class B would be about 1.1 watts (that is per tube so if push pull is used the output of the whole amplifier could be about 2.2 watts.)  A significant increase over what it was capable of in class A.

 

The class AB amplifier must also be used in push pull when operated at audio frequencies.  When driven with a small signal the AB amplifier, which is biased so the active device is not in cutoff, will actually run as a class A amplifier.  When driven with a large enough signal this amplifier go beyond cutoff level thus running less the 360º but more then 180º.

 

The benefit of an AB configuration over an A is the AB has better efficiency (more then 25% but less then 50%).  The little 6C4 would be able to have a DC input of about 1.65 watts with an output power per tube of about .58 watts.

 

Now the class C amplifier is, of course, biased well beyond cutoff so it operates for less then 180º.  The operating time, though it varies some, is more in the range of 45º or less.  If a signal is of to low a level it will not be sufficient to drive a class C amplifier so there will be no output, there will also be no DC current, until it is increased to a sufficient level to overcome the bias and drive the amplifier into conduction.  In the case of the 6C4 it would only be conducting during the positive peaks of the signal.  If the signal was small enough the output could be less then the input but as the signal is increased the output will increase in a non linear manor.  If you double the drive the output will be well more then double.  The class C amplifier will run at 75 to 90 percent efficiency.   It can also be run at higher power so the 6C4, using the same voltage as before, can not run about 4 watts and have an output of above 3 watts.  When operating in class C the 6C4 can safely be operated with 300 volts plate voltage at .025 amps which equals 7.5 watts DC input.  That means it can have an output of around 6 watts or more.

 

A class C amplifier puts out a full sign wave even though it is only conducting on a single peak.  While a vacuum tube that would be only on during the positive peaks transistors may turn of during the positive or the negative peak depending on the type of transistor.

 

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