Slow down smu, if you want a +/-50V swing (100Vp-p) at 20MHz, that'll require a slew rate of S = 2pi f A = 6280V/us, which is *not* easy. If the load includes some capacitance, even 6-feet of coax, you'll need an output-current capability of i = S C = 6.3E9 * 180pF = 1.1A peak. Even if your peak-peak need is less than 100V, you may still find one of these parameters giving you trouble. At any rate, you may be looking at a fairly-serious RF power amp.
How linear? Whats the gain got to be? What's the load? Whats the bandwidth?
It only requires a few tens of MHz to get to 1000V/uS. A fairly simple common source amplifier using a fast MOSFET could do this.
Hello,
I search the best structure about an amplifier that can perform:
If someone has some suggestion or some schematic, it would be great.
smu
Hello,
I search the best structure about an amplifier that can perform:
If someone has some suggestion or some schematic, it would be great.
Thank you in advance
smu
This point is not so critical.
The gain must be 8.
The load is a 400 ohms resistor.
A bandwidth of 20 MHz is enough.
Good idea.
Thank you
smu
This is looking like:
ASCII art:
+500V ! \ / \ 1K / ! !!-+------- Output -----!! !!- ! / \ 33R / ! -100VMOSFET on:
Drain side Vth = 500 * 400/(1K + 400) = 143V Rth = 286
I = (143 + 100) / (286 + 33) = 0.76A
P(33) = 0.76^2 * 33 = 19W
P(1K) = (500 + 100 - 33*0.76)^2/1K = 391W
The MOSFET will have to be quite big or perhaps a few in parallel.
400//1K with Cds must be above 20MHz so:20MHz < 1 / (2*PI*286*C)
C < 1 / (2*PI*286*20MHz)
C < 27.8pF
That's trouble brewing!
We can
(1) lower the resistances and boil a lot of water. Since water is nearly free and I like tea, I'll suggest you consider this first.
(2) Place a series RC combination across the 33R to put a compensating zero in. You need to know the characteristics of a proposed MOSFET. It may not be posible.
(3) Add feedback and more gain. Complexity will grow quickly if you must stay discrete.
(4) Add a peaking coil. This like the RC again may not be enough.
(5) Use tricky cascoding because the 1000V/uS limits the 20MHz amplitude. This is too messy.
Another suggestion. Have a a look at some APEX amplifiers. There is at least one that does 1000V/us.
Not cheap but possibly quicker useable than a selfbuilt one.
Rene
I said 20 MHz but I need only 2 MHz so I think I need 1257 V/µs. If I have understood the preceding information, the output current capability must be 80 mA for a 60pF load.
smu
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