RF transistor biasing

[Jamie Morken]

Hi,

Why do many RF amplifiers use an external biasing resistor instead of a
push pull halfbridge configuration? Is the external biasing main
function to pull up the output since the amplifier only can sink current
on its output?

example of an RF amplifier that requires external biasing:
"http://www.minicircuits.com/appnote/an60010.pdf"

Also can the same MMIC (monolithic microwave integrated circuit)
amplifier be used either as a LNA or as an RF power amplifier?
ie. the "Gali series":
"http://www.minicircuits.com/dg03-166.pdf"

cheers,
Jamie

 

[Don Pearce]

On Thu, 30 Dec 2004 07:16:22 GMT, Jamie Morken <jmorken@shaw.ca>
wrote:

Hi,

Why do many RF amplifiers use an external biasing resistor instead of a
push pull halfbridge configuration? Is the external biasing main
function to pull up the output since the amplifier only can sink current
on its output?

example of an RF amplifier that requires external biasing:
"http://www.minicircuits.com/appnote/an60010.pdf"

Also can the same MMIC (monolithic microwave integrated circuit)
amplifier be used either as a LNA or as an RF power amplifier?
ie. the "Gali series":
"http://www.minicircuits.com/dg03-166.pdf"

cheers,
Jamie

Several reasons. It saves a pin and lets you use a four-pin package.
It provides flexibility to use whatever rail voltage you have. And it
removes one of the bigger power dissipation components from the
package.

d

Pearce Consulting
http://www.pearce.uk.com

 

[John Larkin]

On Thu, 30 Dec 2004 07:16:22 GMT, Jamie Morken <jmorken@shaw.ca>
wrote:

Hi,

Why do many RF amplifiers use an external biasing resistor instead of a
push pull halfbridge configuration? Is the external biasing main
function to pull up the output since the amplifier only can sink current
on its output?

example of an RF amplifier that requires external biasing:
"http://www.minicircuits.com/appnote/an60010.pdf"

Also can the same MMIC (monolithic microwave integrated circuit)
amplifier be used either as a LNA or as an RF power amplifier?
ie. the "Gali series":
"http://www.minicircuits.com/dg03-166.pdf"

cheers,
Jamie

The technologies only produce fast N-type devices... silicon or InGaAs
NPN bipolar transistors so, yes, they can only pull down. There have
been a few MMIC-type amps with active pullups (NEC maybe? Maxim?) but
they tend to be slow.

There's nothing inside most of these parts but a Darlington and a
couple of resistors.

We had a thread a few weeks ago on the problems associated with
creating a super-wideband inductor for use in the pullup.

John

 

[Rene Tschaggelar]

Jamie Morken wrote:

Hi,

Why do many RF amplifiers use an external biasing resistor instead of a
push pull halfbridge configuration? Is the external biasing main
function to pull up the output since the amplifier only can sink current
on its output?

example of an RF amplifier that requires external biasing:
"http://www.minicircuits.com/appnote/an60010.pdf"

push pull ? They'd to be driven anti phase. Can you supply
antiphase drive over the mentioned bandwidths ? From basically
DC to several GHz ?

Rene
--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net

 

[Active8]

On Thu, 30 Dec 2004 07:50:07 -0800, John Larkin wrote:

We had a thread a few weeks ago on the problems associated with
creating a super-wideband inductor for use in the pullup.

what was the subject if you remember?


--
Best Regards,
Mike

 

[John Larkin]

On Thu, 30 Dec 2004 14:56:11 -0500, Active8 <reply2group@ndbbm.net>
wrote:

On Thu, 30 Dec 2004 07:50:07 -0800, John Larkin wrote:



We had a thread a few weeks ago on the problems associated with
creating a super-wideband inductor for use in the pullup.


what was the subject if you remember?

"Wideband inductors", Dec 14.

Oh, one of my guys did disect a PSPL bias tee...

John

 

[Jamie Morken]

Hi,

Andy wrote:

IM specs aren't great. But, if your specs aren't too
stringent, it's very handy to plop one or two down on
a 100 mil track on G-10 (FR4) to get 50 ohms, and move on...

Is that for 0.062" thick PCB? So you can run 100mil traces for all the
RF signals if everything is 50ohms? Does this apply to a 2layer
board with a ground plane on the bottom?

They can even be directly paralleled to almost double the
power output (but not the gain).

Do you use a common bias resistor/wideband inductor for this or one per
amplifier?

cheers,
Jamie

 

[John Miles]

In article <b0q8t05t0ql52qd3g9mpug4dmtdo6as50k@4ax.com>,
jjlarkin@highSNIPlandTHIStechPLEASEnology.com says...

"Wideband inductors", Dec 14.

Oh, one of my guys did disect a PSPL bias tee...

John

Have you seen the Mini-Circuits ADCH-80A? Nice wideband part with
multiple series inductances, characterized to (IIRC) 8 GHz.

Might have been mentioned in the other thread; I didn't see that one.

-- jm

------------------------------------------------------
http://www.qsl.net/ke5fx
Note: My E-mail address has been altered to avoid spam
------------------------------------------------------

 

[John Larkin]

On Fri, 31 Dec 2004 10:17:59 -0800, John Miles
<jmiles@pop.removethistomailme.net> wrote:

In article <b0q8t05t0ql52qd3g9mpug4dmtdo6as50k@4ax.com>,
jjlarkin@highSNIPlandTHIStechPLEASEnology.com says...

"Wideband inductors", Dec 14.

Oh, one of my guys did disect a PSPL bias tee...

John

Have you seen the Mini-Circuits ADCH-80A? Nice wideband part with
multiple series inductances, characterized to (IIRC) 8 GHz.

Might have been mentioned in the other thread; I didn't see that one.

Those are nice, but L is low and drops like a rock as the DC current
increases. For the amp we're doing now, we needed 100 uH maybe at 200
mA, so we wound up with a string of three ferrite beads of increasing
size, followed by two axial inductors, each with a shunt resistor.
Inductors are sure non-ideal gadgets.

John

 

[John Larkin]

On 2 Jan 2005 06:41:01 -0800, "Andy" <andysharpe@juno.com> wrote:

******** 100 mils on .062" FR-4 , ground one side, track on
the other, forms a 50 microstrip (very very close). I have
often just used an exacto knife to cut it out.
I have also used adhesive copper tape on the ungrounded side.

This is
known as "micro-strip" and is well defined by graphs of
Wheeler's Equations. It's just as easy to make 35 ohm or
70 ohm, but I forget the widths right now.... Higher impedance
means narrower track, and by 300 ohms the track is smaller
than I want to mess with using an exacto knife.

Note that this subject is a complex one, but 50 ohm track
is so common that RF engineers, who do their own assembly and
building (getting rarer these days) remember certain dimensions as
"rules of thumb".....
While the mathematical types will argue till hell freezes
over about 2 or 3 ohms difference, as a practical matter it
is immeasureable in performance....

Agilent's free AppCad program is great for calculating microstrip,
stripline, and cpw impedances.

John