4076	2010-01-24 09:01:27	k5nwa	Grounded Base Power amplifiers
Message	Date	From	Subject
One very seldom sees a grounded base power amplifier in the many radios available in kits or articles, are there technical reasons why not, or is it lack of examples? With tubes it's quite popular, and I can see some advantages to using a grounded base (grounded at RF) Thanks Cecil k5nwa www.softrockradio.org www.qrpradio.com < http://parts.softrockradio.org/ > Never take life seriously. Nobody gets out alive anyway.
4078	2010-01-24 09:24:22	Chris Trask	Re: Grounded Base Power amplifiers
> > One very seldom sees a grounded base power amplifier in the many > radios available in kits or articles, are there technical reasons why > not, or is it lack of examples? > > With tubes it's quite popular, and I can see some advantages to using > a grounded base (grounded at RF) > It primarily has to do with stability, and the inductance of the bond wires to the base is the main culprit. Soviet radar designers loved grounded-base Class C amplifiers back in the 60s and 70s, and perhaps even now. Motorola at one time made a dual grounded-base power transistor where the base metalization was actually the bottom side of the die. Chris ,----------------------. High Performance Mixers and / What's all this \ Amplifiers for RF Communications / extinct stuff, anyhow? / \ _______,--------------' Chris Trask / N7ZWY _
4079	2010-01-24 10:13:39	w4zcb	Re: Grounded Base Power amplifiers
primarily has to do with stability, and the inductance of the bond wires to the base is the main culprit. Soviet radar designers loved grounded-base Class C amplifiers back in the 60s and 70s, and perhaps even now. Motorola at one time made a dual grounded-base power transistor where the base metalization was actually the bottom side of the die. Chris Alas, also ran afoul of the same problem building a good post mixer feedback amplifier back in the early 90's. I had tried to match Doug DeMaws experience with the VMP-4 as a mixer back in the 70's, and had to order a dozen of the VMP-4's to get my vendors attention. Alas again, things didn't turn out quite that way, and even Oolrich Rohde wound up having troubles with the circuit. Seemed as though the results were less than repeatable. Fast forward 20 years, and we were looking for a low noise post mixer amp and I chose Haywards neither fish nor fowl circuit of the feedback FET amp as in IRFD fig 6.11 using the VMP-4. Made a wonderful amp. Noise figure 1.1 dB, and an excellent IMD performer. Should have been, 28 Volts at 700 mA! We tried other power FETs since the VMP-4 was obsolete, and couldn't repeat the performance. Tore apart some MA/COM Fets that had similar specs to a VMP-4 and quickly found the reason why. Siliconix had TWELVE paralleled source wire bonds to the source bridge. The MA/COMs (DU2810s and 2820's) and power FETS from several other sources had TWO. Had chats with Ed Oxner about it all, and he remarked that they had operated the VMP-4's with the "lids" off and could observe the source wires vibrating. They added source wires till the vibrations quit. The bean counters for the other companies always quit at two. That circuit demands a low source impedance. All documented in old RadComs by Pat Hawker. Gave up the entire idea in favor of the quad J-310 amplifier in the same circuit. 12 Volts at 70-80 mA, when realized that the VMP-4's were perfectly capable of blowing everything after it since if you tuned accidently across a strong signal it could accomodate 6 Watts out to the filter! There's such a thing as too much of a good. W4ZCB [Non-text portions of this message have been removed]
4080	2010-01-24 10:26:57	k5nwa	Re: Grounded Base Power amplifiers
At 11:24 AM 1/24/2010, you wrote: > > > > > > One very seldom sees a grounded base power amplifier in the many > > radios available in kits or articles, are there technical reasons why > > not, or is it lack of examples? > > > > With tubes it's quite popular, and I can see some advantages to using > > a grounded base (grounded at RF) > > > >It primarily has to do with stability, and the inductance of the bond >wires to the base is the main culprit. Soviet radar designers loved >grounded-base Class C amplifiers back in the 60s and 70s, and perhaps even >now. Motorola at one time made a dual grounded-base power transistor where >the base metalization was actually the bottom side of the die. > >Chris > Even RF transistors? I would assume they provide better bonding that your average transistor. And how about with negative feedback such as a Norton amplifier? The seem to do well for low power. Cecil k5nwa www.softrockradio.org www.qrpradio.com < http://parts.softrockradio.org/ > Never take life seriously. Nobody gets out alive anyway.
4081	2010-01-24 17:06:10	ajparent1	Re: Grounded Base Power amplifiers
I have a bunch of devices including SD1565s 500W uhf pulse power pair. I may make a 2M amp using them (class C fm). When derated to 100% duty cycle power then should do more than a few watts. Being comm
4084	2010-01-24 19:27:02	KK7B	Re: Grounded Base Power amplifiers
You could play with the technique I used in those QST experimental linear amplifiers a few years ago, connecting to the transistor with a pair of ferrite sleeve baluns right at the device, so you drive and load each individual transistor differentially. DC Float the heat sink, but let it look like RF ground since the heat sink would have a lot of capacitance to "real" ground. It might work, and the DC and RF connections would be independent so you could apply conventional current mirror bias. The technique is a bit of a conceptual stretch to wrap your mind around--a bit like a cavity amplifier with the high voltage connection to a 3CX100A5 plate on the cold outside of the cavity and all the RF on the inside... It would take courage to try with a 28v 40A power supply. It works great with 28v at 1A though, and I can RF ground any of the three transistor leads without affecting the amplifier. The whole thing really is floating in the middle of a pair of transmission lines. Fun to think about. Rick
4086	2010-01-27 04:37:43	davidpnewkirk	Re: Grounded Base Power amplifiers

4087	2010-01-27 05:23:31	Chris Trask	Re: Grounded Base Power amplifiers
> > > With tubes [the grounded-grid configuration is] quite popular, and > > I can see some advantages to using a grounded base (grounded at RF) > > One great difference between a grounded-grid vacuum-tube amplifier > and any transistor analog of it is the grounded grid of a tube acts > as an electrostatic shield and the grounded base/gate of a transistor > doesn't. > > This is why, even with the best possible gate grounding the (in my > opinion, overused in ARRL publications), grounded-gate JFET RF > amplifier is often far from unconditionally unstable. Significant > capacitive input-to-output coupling, somewhat reduced by shunt > channel-to-gate capacitance to ground, always exists in it. Here > is one instance where the vacuum-tube analog to its later solid-state > counterparts is (in this respect) better. > <> For small-signal common-base (CB) bipolar amplifiers, there's a very interesting design trick for wide-band neutralization that I found in a very obscure text on transistor circuit design. You begin with an N:1:1 (or N:2CT) tranformer. One secondary arm is connected to the transistor emitter. The opposite arm has a resistor to ground and a capacitor to the transistor collector. When the resistor value is the same as the transistor base spreading resistance (Rbb) and the capacitor is the same as the transistor collector-to-base capacitance (Ccb), the amplifier is neutralized almost to the transition frequency of the transistor (fT). I modeled this on PSpice and built a couple of circuits to verify that it works. Chris ,----------------------. High Performance Mixers and / What's all this \ Amplifiers for RF Communications / extinct stuff, anyhow? / \ _______,--------------' Chris Trask / N7ZWY _
4088	2010-01-27 11:28:54	davidpnewkirk	Re: Grounded Base Power amplifiers

4089	2010-01-27 12:07:21	Johan H. Bodin	Re: Grounded Base Power amplifiers
Interesting... I am trying make a mental picture of the circuit, is the primary winding used as input? Is the C.T. "AC grounded"? 73 Johan SM6LKM Chris Trask wrote: > For small-signal common-base (CB) bipolar amplifiers, there's a very interesting design trick for wide-band neutralization that I found in a very obscure text on transistor circuit design. You begin with an N:1:1 (or N:2CT) tranformer. One secondary arm is connected to the transistor emitter. The opposite arm has a resistor to ground and a capacitor to the transistor collector. When the resistor value is the same as the transistor base spreading resistance (Rbb) and the capacitor is the same as the transistor collector-to-base capacitance (Ccb), the amplifier is neutralized almost to the transition frequency of the transistor (fT). I modeled this on PSpice and built a couple of circuits to verify that it works.
4090	2010-01-27 12:22:43	Chris Trask	Re: Grounded Base Power amplifiers
> > Chris Trask wrote: > > For small-signal common-base (CB) bipolar amplifiers, there's a very > > interesting design trick for wide-band neutralization that I found in a very > > obscure text on transistor circuit design. You begin with an N:1:1 (or N:2CT) > > tranformer. One secondary arm is connected to the transistor emitter. The > > opposite arm has a resistor to ground and a capacitor to the transistor > > collector. When the resistor value is the same as the transistor base > > spreading resistance (Rbb) and the capacitor is the same as the transistor > > collector-to-base capacitance (Ccb), the amplifier is neutralized almost to > > the transition frequency of the transistor (fT). I modeled this on PSpice > > and built a couple of circuits to verify that it works. > > > Interesting... I am trying make a mental picture of the circuit, is the > primary winding used as input? Is the C.T. "AC grounded"? > > 73 > Johan SM6LKM > Yes to both. It's actually best if the transformer be 1:1:1 (1:2CT) so that you can twist three wires together and get better coupling, then do an impedance transformation before that. Reason is that due to the low impedance of the emitter good coupling will be essential so as not to suffer significant losses. I learned this last year when designing loop antenna matching networks. Also, stay away from ferrite materials at these low impedances as they tend to be very lossy. Powdered iron is much better. Chris Trask N7ZWY WDX3HLB Senior Member IEEE http://www.home.earthlink.net/~christrask/ Chris ,----------------------. High Performance Mixers and / What's all this \ Amplifiers for RF Communications / extinct stuff, anyhow? / \ _______,--------------' Chris Trask / N7ZWY _