Forums

HF design approach

Started by lerameur February 6, 2014
Hello to all,
I am designing an amplifier for frequencies between 500khz to 5Mhz. (+/-40V=
) a couple of watts.
I am a newbie to HF frequencies , but I would like to know what is the best=
 amplifier technique for these frequencies.=20
At first I wanted a pushed and pull mosfet driven by a mosfet driver, but t=
he more I am looking the more i'm confused. I've seen some expensive specia=
lized mosfet at over 400$.... other circuits with  inductors and RF type tr=
ansformers. which path is the way to go in my case. I am a bit lost in this=
 sea of different circuits.
Thanks

Ken
On Thursday, 6 February 2014 20:50:46 UTC-5, lerameur  wrote:
> Hello to all, >=20 > I am designing an amplifier for frequencies between 500khz to 5Mhz. (+/-4=
0V) a couple of watts.
>=20 > I am a newbie to HF frequencies , but I would like to know what is the be=
st amplifier technique for these frequencies.=20
>=20 > At first I wanted a pushed and pull mosfet driven by a mosfet driver, but=
the more I am looking the more i'm confused. I've seen some expensive spec= ialized mosfet at over 400$.... other circuits with inductors and RF type = transformers. which path is the way to go in my case. I am a bit lost in th= is sea of different circuits.
>=20 > Thanks >=20 >=20 >=20 > Ken
Aloso, should ibe using high or low Rds mosfet? Ken
"lerameur"

Hello to all,
I am designing an amplifier for frequencies between 500khz to 5Mhz. (+/-40V) 
a couple of watts.
I am a newbie to HF frequencies , but I would like to know what is the best 
amplifier technique for these frequencies.
At first I wanted a pushed and pull mosfet driven by a mosfet driver, but 
the more I am looking the more i'm confused. I've seen some expensive 
specialized mosfet at over 400$.... other circuits with  inductors and RF 
type transformers. which path is the way to go in my case. I am a bit lost 
in this sea of different circuits.


** This is the way it is really done:

http://www.thermionic.org/tx807s.jpg





...  Phil 


On Thursday, 6 February 2014 20:50:46 UTC-5, lerameur  wrote:
> Hello to all, >=20 > I am designing an amplifier for frequencies between 500khz to 5Mhz. (+/-4=
0V) a couple of watts.
>=20 > I am a newbie to HF frequencies , but I would like to know what is the be=
st amplifier technique for these frequencies.=20
>=20 > At first I wanted a pushed and pull mosfet driven by a mosfet driver, but=
the more I am looking the more i'm confused. I've seen some expensive spec= ialized mosfet at over 400$.... other circuits with inductors and RF type = transformers. which path is the way to go in my case. I am a bit lost in th= is sea of different circuits.
>=20 > Thanks >=20 >=20 >=20 > Ken
I heard vacuum tube where expensive and did not perform better and also are= very expensive Ken
On Thursday, February 6, 2014 8:50:46 PM UTC-5, lerameur wrote:
> Hello to all, >=20 > I am designing an amplifier for frequencies between 500khz to 5Mhz. (+/-4=
0V) a couple of watts.
>=20 > I am a newbie to HF frequencies , but I would like to know what is the be=
st amplifier technique for these frequencies.=20
>=20 > At first I wanted a pushed and pull mosfet driven by a mosfet driver, but=
the more I am looking the more i'm confused. I've seen some expensive spec= ialized mosfet at over 400$.... other circuits with inductors and RF type = transformers. which path is the way to go in my case. I am a bit lost in th= is sea of different circuits.
>=20 > Thanks >=20 >=20 >=20 > Ken
A few watts so ~100mA of current? If money is no object you could look at = the Apex opamps. http://www.apexanalog.com/ If you want to make your own, then an opamp driving a push-pull would be OK= . I don't know about using Fet's in the push pull. That sounds like more = cross over distortion. I might try some bjt's... (easier for me to bias.) = Will a bit of cross over distortion be bad? Re: Rds of mosfet. Since your load is someting like 400 ohms (what is the = load?) I don't think the Rds of the mosfet will matter that much. =20 George H.
On Fri, 7 Feb 2014, lerameur wrote:

> On Thursday, 6 February 2014 20:50:46 UTC-5, lerameur wrote: >> Hello to all, >> >> I am designing an amplifier for frequencies between 500khz to 5Mhz. (+/-40V) a couple of watts. >> >> I am a newbie to HF frequencies , but I would like to know what is the best amplifier technique for these frequencies. >> >> At first I wanted a pushed and pull mosfet driven by a mosfet driver, but the more I am looking the more i'm confused. I've seen some expensive specialized mosfet at over 400$.... other circuits with inductors and RF type transformers. which path is the way to go in my case. I am a bit lost in this sea of different circuits. >> >> Thanks >> >> >> >> Ken > > I heard vacuum tube where expensive and did not perform better and also > are very expensive >
Not much was ever done with broadband tube amplifiers. They existed, but not out in public view much. Transistors, with their lower impedance, are easier to make broadband. That said, you've not revealed what this is for. The application may reflect on what you need, rather than just assuming "an amplifier that goes from here to there". Michael
On Thursday, 6 February 2014 20:50:46 UTC-5, lerameur  wrote:
> Hello to all, >=20 > I am designing an amplifier for frequencies between 500khz to 5Mhz. (+/-4=
0V) a couple of watts.
>=20 > I am a newbie to HF frequencies , but I would like to know what is the be=
st amplifier technique for these frequencies.=20
>=20 > At first I wanted a pushed and pull mosfet driven by a mosfet driver, but=
the more I am looking the more i'm confused. I've seen some expensive spec= ialized mosfet at over 400$.... other circuits with inductors and RF type = transformers. which path is the way to go in my case. I am a bit lost in th= is sea of different circuits.
>=20 > Thanks >=20 >=20 >=20 > Ken
Hopefully no cross over distortion. I want to replicate Richard Vialle expe= riment about resonating a coil over a tube. http://jnaudin.free.fr/rvproject/html/ugentest27.htm#TESTV132 .. keep in mind..The question here is not if this project (richard vialle) = falls into sanity or not... but to have a 40V amplifer . Regards, K
On Friday, February 7, 2014 2:11:12 PM UTC-5, lerameur wrote:
> On Thursday, 6 February 2014 20:50:46 UTC-5, lerameur wrote: > > Hello to all, > > I am designing an amplifier for frequencies between 500khz to 5Mhz. (+/=
-40V) a couple of watts.
> > I am a newbie to HF frequencies , but I would like to know what is the =
best amplifier technique for these frequencies.=20
>=20 > > At first I wanted a pushed and pull mosfet driven by a mosfet driver, b=
ut the more I am looking the more i'm confused. I've seen some expensive sp= ecialized mosfet at over 400$.... other circuits with inductors and RF typ= e transformers. which path is the way to go in my case. I am a bit lost in = this sea of different circuits.
>=20 > > Thanks > > Ken > Hopefully no cross over distortion. I want to replicate Richard Vialle ex=
periment about resonating a coil over a tube.
>=20 > http://jnaudin.free.fr/rvproject/html/ugentest27.htm#TESTV132 >=20 > .. keep in mind..The question here is not if this project (richard vialle=
) falls into sanity or not... but to have a 40V amplifer .
>=20 > Regards, > K
Well there's a circuit there, start by copying it. (I don't know much about fets at 5MHz.) I might want to put a choke in the power supply line.=20 I didn't translate the french. Does he bias the fet on with the sig. gen.? I was going to say if you don't want cross over distortion, and can handle = the power waste then class A is an easy option. =20 That circuit only needs +40V.. I was thinking +/- 40.=20 In which case its easier to wrap an opamp around the pass element. When I think of these circuits (opamp->class A) I always imagine feeding ba= ck the current, by just hanging a sense resistor to ground. (or high side i= f I must.)=20 But what if I wanted to feedback the load voltage? (What do I sense and wh= ere's the pass element?) =20 George H.
On Thu, 06 Feb 2014 17:50:46 -0800, lerameur wrote:

> Hello to all, > I am designing an amplifier for frequencies between 500khz to 5Mhz. > (+/-40V) a couple of watts. > I am a newbie to HF frequencies , but I would like to know what is the > best amplifier technique for these frequencies. > At first I wanted a pushed and pull mosfet driven by a mosfet driver, > but the more I am looking the more i'm confused. I've seen some > expensive specialized mosfet at over 400$.... other circuits with > inductors and RF type transformers. which path is the way to go in my > case. I am a bit lost in this sea of different circuits. > Thanks > > Ken
There's not enough information in your question. Your stated voltage and power implies load equivalent parallel resistances in the neighborhood of 1k-ohm. How reactive are your loads? Is there any significant inductance? Capacitance higher than about 30pF? If the answer is "yes" then that complicates your task. How much can you expect your load to change? Do you need to drive a variety of loads, or just one? If just one, do the load characteristics vary much from unit to unit, or over temperature or humidity or pressure or as a function of age or in some other manner? Varying loads make the design task harder. How cost sensitive is your application? Are you building ten systems for medical or military? Or are you building a product that needs to ship 100,000 a year and sell for less than the price of a hamburger? Few systems points you towards a $$$ off the shelf solution. Lots of systems points you toward $$$$ spent in custom circuit design with low parts cost. How broadband is your signal? Are you going to be using 500kHz to 5MHz at the same time, or is this a radio-like application where the signal is always some narrow slice of the operating bandwidth at any given time? Broadband is harder. Narrowband is easier. How efficient do you need to be? Can you afford to burn up 20 watts to get "a few" to the load? Or is every wasted erg a tragedy? Efficiency will cost you here, in terms of parts cost and engineering time as well. Every single one of the answers that you can give affects what circuit is "best". As an example, I used to build low-end military stuff (which overlapped with high-end civilian law enforcement and search-and- rescue). We usually expected to build 100 units a year of any given product, with a lifetime for any one board of around five years. So there wasn't much opportunity for payback if you flogged the design and took $50 out of the parts cost. For us, when faced with "+/- 40V, 5MHz top end, a few watts", the first place we looked was the Apex Semiconductor site. Apex built hybrid op-amps that were higher voltage or higher power than you could get in a monolithic. They built really good products that nearly always worked as promised, and they charged up the wazoo. If your application fits the "small volume expensive" mold, then find out who makes those parts now (Apex got bought by someone and I haven't kept track of who). Then look through the catalog and chances are there'll be an amplifier of sufficient voltage capacity and gain-bandwidth product to satisfy your needs. If your application is for 100,000 a month, then talk to Jim Thompson about building you a custom IC that you can pair with a couple of $0.10 off-board MOSFETs. He'll charge you an arm an a leg for the work, and the fab he points you to will charge that much, too, but when you amortize the cost over your production volume it'll be fingernail pairings. If your application is somewhere in the middle -- well, you need something in the middle, ranging from an all-transistor custom design, to a really fast op-amp driving a couple of outboard transistors as a final amp, to just about anything. -- Tim Wescott Control system and signal processing consulting www.wescottdesign.com
On Thursday, 6 February 2014 20:50:46 UTC-5, lerameur  wrote:
> Hello to all, >=20 > I am designing an amplifier for frequencies between 500khz to 5Mhz. (+/-4=
0V) a couple of watts.
>=20 > I am a newbie to HF frequencies , but I would like to know what is the be=
st amplifier technique for these frequencies.=20
>=20 > At first I wanted a pushed and pull mosfet driven by a mosfet driver, but=
the more I am looking the more i'm confused. I've seen some expensive spec= ialized mosfet at over 400$.... other circuits with inductors and RF type = transformers. which path is the way to go in my case. I am a bit lost in th= is sea of different circuits.
>=20 > Thanks >=20 >=20 >=20 > Ken
Hi, This is a lot of detail. Actually, I am only doing an experiment, so I do n= ot want to spend a lot of money. My load is about 3uH and 550pF. Actually I= want an amplifier with no crossover distortion. So my question is, what is= the best economical way to do an amplifier with no crossover distortion.? Ken