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# How to figure about the class of amplifier from its operating voltage and current?

Jay Sharma

7/20/2010 6:02 AM EDT

I am wondering how can we determine the class of amplifier just by looking at its operating voltage and current.

kinnar

7/20/2010 3:17 PM EDT

Generally if it takes current in mA then it is a Preamp or Voltage Amplifier, that will generaly class A.
If it takes current in terms of Amps, then it can be a Power Amplifer, that may be Class B, Class AB, Class C or Class D.

Bhola_#1

7/24/2010 2:20 PM EDT

I am not sure if I understand your comment properly. My question was how can we determine class of amplifier from its operating current and voltage if we know PA power such as 1W, 2W etc.) I do understand that Amp drawing more current ( per amplfier class definition) will behave as class A as compared to class AB or D. I have seen many PAs in wireless world say in basestation applications, drawing current in mA but behaving as class A.

7/20/2010 4:46 PM EDT

you might also wanna look at efficiency to figure out the class of amp.

B.V.Rao

7/21/2010 5:12 AM EDT

If quiescent current (without any input) and current drawn when driven to full output are within 1:2 it is likely to be a Class A amplifier (50% efficiency). If the ratio is higher then it is AB or D.

Bhola_#1

7/24/2010 2:29 PM EDT

Thanks for your comment. Basically, if PA draws double the current at saturation level say Psat/P1dB, it is class A with least efficiency ie 50%. I am wondering if we donot know Icc@ P1dB of the device, just Icq, V and power. For example, say HBT 1W PA with 120mA, +5V. How can we determine what class it come under? As per my knowledge, DC power (approx.) for this PA would be 2(RFpower) ie 2W...from here we can somehow find class of PA from definition such as 50% efficiency and stuff. Please let me know if this makes any sense.

Dan Auton

7/24/2010 11:37 AM EDT

Between vacuum tube and solid state, yes!

Bhola_#1

7/24/2010 2:30 PM EDT

Dan

Dan Auton

7/24/2010 11:36 PM EDT

It was a little tongue in cheek as a tube amp will have a 300V+ power supply so easy to spot. There are more amp classes in audio than RF so the range is less but the link below is to an IEEE article which says that there are 10 classes of RF amps. Class-D requires greater than 2x switching speed vs. the lowest frequency so not a good fit for RF. But there are class G and H RF amps where the voltage rails are varied vs. the input signal so a low voltage rail at low signal level. GSM RF PA's run at high efficiency as the modulation is all phase controlled so around 50% but Edge, 3G, CDMA all do amplitude modulation so 30% is good for a AB and class G/H would be be over 50 very hard to do and keep low distortion. Good luck

Bhola_#1

7/25/2010 11:20 PM EDT

Thanks Dan for your informative comment. Yes, efficiency is a big factor since these devices goes into infrastrucure applications such as driver stage or HPA where efficiency as well as linearity plays a big role. Have you heard or worked with class F or inverse F amp. It is becoming big piece of research in RF field. You mentioned WCDMA, EDGE with AM. I am not sure if I have seen any 3G application specifically in network infrastructure field using AM. It is basically PSK (phase modulation).