Drain Current

[Tiaotiao]

Hello Everybody:
I just found out this forum and it is extremely useful for my design! By simply going through the articles and messages posted here, I have gained a lot of valuable information. Thanks to everybody!
Right now, I am working on a 20 dBm input, 33 dBm output at 150 MHz with bandwidth 30 MHz and Vcc = 15V class E power amplifier design. My schematic and the component values are obtained from the Sokal article and simulation is done using Microwave Office software. The output seems to be reasonable except that the drain current in time domain goes to negative. I can not understand this. The model used is MRF 9060SR1. Can this be caused by the SPICE parameters associated with the simulation model? Sokal's artical dose not provide a sample driver circuit design. Where could I find this information? In addition, where could I learn the method to make air-wound transformers at 150MHz with any impedance ratio?
THANKS A LOT!!!

Tiaotiao Xie
2005, 06, 16

[frank carcia]

Hi,
I suspect the amplifier will not work in class e mode over that wide of a bandwidth. I suspect your shunt cap value is too low allowing the drain voltage to swing above the vcc reversing the current flow. Check your drain inductor value and drain bypass cap value. Try to get the amplifier to work over 100 KHz first. Look at the ARRL handbook for air core inductors.
This may reference ARRL VHF publications to use as a reference.
My 160 meter final at 1.9 mhz can cover about 50 KHz without retuning.
good luck I have seen other simulations vary efficiency a lot over a small change in frequency. gfz

[Tiaotiao]

Hello Frank:
Thank you very much for your help! However, I am confused now. Is the shunt capacitor the one that is placed in parallel with the transistor on right hand side in Sokal's article? Also, is the drain inductor you mentioned the RF choke? Which capacitor in Sokal's article is the drain bypass capacitor? I guess that he did not include one in the basic schematic of class E power amplifier. Is it right? Thanks again!

Tiaotiao Xie
2005, 06, 17

[frank carcia]

shunt= collector / drain to ground cap
RF choke is drain inductor
bypass cap is required on B+ side of RF choke to ground to isolate the power supply from the amplifier. This has to be low reactance at 150 mhz.
look at WA1QIX classe web site.
I still don't think you can get 30 MHz. BW at 150 MHz and remain in class E mode. Sounds like a class A amplifier or maybe push pull AB.

[Tom WA3KLR]

Tiaotiao,

About the drain current going negetive at the start of the transistor conduction:

This is never talked about, but I think is a fundamental of Class E operation.

I have looked at my simulations of high power 4 MHZ. power amplifiers and this small area of negetive drain current just when the transistor turns on is very small. It does tend to correlate with what the main tank circuit (l2 & C2) fly-wheel current is doing however. I think that this is where the negetive drain current comes from. The tank circuit current does change polarity shortly after the transistor turns on.

I simulated your PA situation and this negetive drain current effect is more pronounced. The negetive drain current curve does change smoothly and matches to the positive drain current function without a pertubation at the zero cross-over. I have tried to minimize the effect, but so far I am at a loss to explain why it is more pronounced in your VHF low power circuit.

But I would say "not to worry".

You stated your application bandwidth as 30 MHz. I assume then that your frequency range is 135 - 165 MHz. What is the allowable dB variation for your application?

I simulated your circuit with a design Q of 4 and the output was highest at 135 MHZ and continually dropping as the frequency went to 165 MHz. 135 MHZ output was 2 dB above 150 MHz. 165 MHZ output was -1 dB from 150 MHz.

Unlike other classes of tuned amplifiers, the properly-tuned PA does not loose power like you would think as the conditions change. I found that (staying on one frequency) and de-tuning the series tank capacitor either side of the properly tuned point did this:
On one side - the input power was increasing, output power increasing, with a slight lowering of efficiency.
On the other side - the input power was decreasing, output power decreasing, with a slight increase of efficiency.

[Tiaotiao]

Hello Tom:
Thank you very much for your help!
In my simuation, I measured the current waveform going through series C2 and L2. It swings between a positive and negative values. Thus, I also think that this is the major problem that causes drain current to go below zero. However, the time duration for negative current is about 33% of one period after optimizing the circuit to meet the design goals. It is unresonably high.
Both the highest efficiency and the highest output power occur right at 150 MHz. For output power, the value at 165 MHz is almost 2 dB less than that at 135 MHz, with peak as 38.14 dBm. The highest efficiency is 88.31% with 73.32% and 70.68% at both sides of bandwidth, respectively. I used Q = 10 in my calculation and the circuit values after optimization are listed as below:

C1 = 28.66 pF
RF choke = 3815 nH
Vcc = 15 V
C2 = 46.4 pF
L2 = 40.74 nH
Rload = 8.952 ohm

My simulation tool is Microwave Office. Could you please simulate the circuit with above values to see whehter the results are the same? Especially, the drain current?

Thank you!

Tiaotiao Xie
2005, 06, 17

[Tiaotiao]

Hello Frank:
Thanks again for your reply. Yes, the design might not work for such a wideband. So, I am working on the case you suggested last time. I am desgining a class E power amplifier at 100 kHz with a bandwidth 20 KHz, and 20 dBm input power to get at least 33 dBm output power. The transistor model is MRF 9060SR1.
I just finished reading the article at WA1Q1X Class E website. It is very helpful for my understanding in this subject! However, I got several questions while reading it.
What is the flyback effect?
How to make either a capacitance or an inductance as part of a resonant circuit? This question is regarding with the internal capacitance of the transistor.
What are heavy wires?
It mentions several times that the gate drive must be at least +12 V or 24 V peak to peak. Is this for every class E power amplifier design regardless of the specifications? Or only for modulation purpose?
I also noticed in the basic class E schematic in the website, there is a capacitor at the right most end of the circuit right before the output. But, there is no such a capacitor in Sokal's article. What is it for?
Last, I can not understand the following sentence: "at resonance, the inductive and capacitive reactance of the input inductor and the series capacitor will be approximately equal". Where are the input inductor and the series capacitor in the schematic?

Thanks!

Tiaotiao Xie
2005, 06, 17

[Tom WA3KLR]

Tiaotiao,

I simulated with your values and I see the power peaked much lower in frequency. I had to decrease the C2 value from 46 pf to 41 pf. I still see the highest output at 135 MHz however and decreasing at 150 and more at 165 MHz.

My Linear Technology SwitcherCADIII simulator is not a microwave simulator. I use a behavioral switch for the transistor, not a transistor model. My PA designs use FETs. So I model the FET with the switch resistance set to the RDS(on) of the FET.

With my Class E Design spreadsheet I did finally arrive close to your L and C values and R, but I had to specify a Q of 4.3 and an output power of 10.43 Watts.