RF amp with BJT single-stage

Hi, I'm designing a simple single-stage amp through a BJT in common-emitter config.

On several reference designs observed in the internet, I see that a resistor is used between collector and V_CC, whereas for other designs, an RF choke is used instead.

I'm getting the idea that the resistor based designs implement a class A amp, for linear (AM) amplification, whereas the RF choke is for class C, narrowband amplification.

Is this right?

Thanks RM

Reply to
riccardomanfrin
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On a sunny day (Fri, 20 Sep 2013 01:17:24 -0700 (PDT)) it happened snipped-for-privacy@gmail.com wrote in :

No

;-)

Reply to
Jan Panteltje

Can you tell me more about? Or suggest clarification by my side? R

Reply to
riccardomanfrin

IMHO the choke is there only for noise isolation purposes. It prevents your amplified RF to sneak into the power rail and vice versa, from the power rail to the collector circuit. If it is not a tank circuit, then only its impedance matters, i.e. no resonant amplification etc. I think both of your circuits are class A.

BTW, why don't you use a MAR? Much easier to get a decently working design.

Best regards, Piotr

Reply to
Piotr Wyderski

Hum.. but the choke, in DC analysis, moves the collector voltage to be equi valent to V_CC. Isn't this like moving the quiescent point very close to th e cut-off region? And isn't this going to mean that if I feed with a signal , only about half of it is going to be amplified, which is exactly the defi nition of class C?

At least this was my understanding from Figure 1.21 of this document:

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(please take the burden).

If you mean using an IC, the motivation behind the choice of discrete compo nents is exacly for the same reason why I'm posing this question: because I 'm ignorant about the subject and would like to understand more about it (a nd to this extent discrete design is an easier starting point).

Reply to
riccardo manfrin

riccardo manfrin pisze:

You mean ONLY a choke? Then indeed, it's not class A. I thought it was a resistor in series with a choke. But class C in an input amplifier? C'mon... :-/

Yes, a 4 pin ready-made RF amplifier, e.g. MAR-6.

OK.

But may be bitchy to do it properly.

Best regards, Piotr

Reply to
Piotr Wyderski

Il giorno venerdì 20 settembre 2013 11:30:20 UTC+2, Piotr Wyderski ha scr itto:

equivalent to V_CC

This is a relief. I thought I was getting it all the way wrong.

If you are posing the doubt that such amplifier cannot amplify AM stuff (au dio), well, I totally agree with you! This is going to be for strictly narr owband input signals, high efficiency amplification, as by definition for C lass C.

I expect so :P

Thanks for giving me confirmation! R

Reply to
riccardo manfrin

Nah, you can have whatever bias current you want. Toobz have been doing it for about a century (which... these days, I think that's actually true?). Emitter bias doesn't simply go away. :)

What it does is give you extra VCC for free, doubling your voltage swing for no cost in power. Theoretical maximum efficiency jumps from an embarassing 12.5% to a passable 25% (assuming continuous current, i.e., class A operation).

Beware the extra gain you get from a choke load; obviously, whatever load is attached will load it down, but if you're using a light load (say, preamp, IF stage, etc.), just something to beware of.

Tim

--
Deep Friar: a very philosophical monk. 
Website: http://seventransistorlabs.com
Reply to
Tim Williams

"riccardo manfrin"

Hum.. but the choke, in DC analysis, moves the collector voltage to be equivalent to V_CC. Isn't this like moving the quiescent point very close to the cut-off region?

** No.

And isn't this going to mean that if I feed with a signal, only about half of it is going to be amplified, which is exactly the definition of class C?

** No.

Using a choke allows the signal to swing ABOVE the DC rail - IOW it doubles the available swing.

FFS - read up on class A valve stages that use coupling transformers.

... Phil

Reply to
Phil Allison

"riccardo manfrin"

** Classic usenet scenario.

One damn fool mis-informs others.

... Phil

Reply to
Phil Allison

On a sunny day (Fri, 20 Sep 2013 01:24:21 -0700 (PDT)) it happened snipped-for-privacy@gmail.com wrote in :

Of course drawing a simple ASCII? circuit prevents misunderstandings. But a RF choke in the collector (or drain) does NOT imply a class C output stage.

For example it can be used to get simply more Vce, (same as supply), and then be followed by a capacitor coupled tuned circuit. + | choke Pi filter |---||---- L------- antenna c | | b === === e | | | /// /// ///

This will work fine in class B too. In fact A, B, C has to do with the opening angle, the part of the sinewave where the output stage is 'on'.

If you talk broadband amplifiers then you sometimes see some L in series with the collector resistor, this was common in video output amps (CRT drive) to get some peaking together with the Cce and the Cload (resonance).

There are as usual a thousand configurations possible, so a circuit diagram helps.

An other very common 'inductor in output' is the class A output transformer configuration...

  • | ----- )|| ( []< speaker )|| ----- | transformer c b e | /// In this case the transformer has an air gap in the core to avoid saturation by the DC flowing in the collector. Ofcourse the primary, in case of an ideal transformer, looks resistive, but ideal transformers do not exist, and sometimes people disconnected the speaker, and then you get nice high voltages over such an inductor...

etc etc etc

Reply to
Jan Panteltje

No.

The resistor is necessary if the design needs to work all the way down to DC, but for RF it reduces the available voltage output of the amplifier and burns up power you'd like going into your following circuit.

If you use a choke that has at least 4 times the impedance of the following load, from your lowest frequency of interest to your highest, then you get more power gain, less noise, lower power, etc. With care, the choke will be good for at least a decade, or more.

Rather than trying to learn this stuff by copying things off the net, I suggest you get some books:

"Experimental design for the Radio Amateur" (I think that's the right name) by Hayward (I know that's the right name).

"Radio Frequency Design" by Hayward.

"The ARRL Handbook", just about any year from 1980 to the present.

I think the ARRL has more publications than that -- if you're more an analytical engineer type, trained in analog circuits, you'll want "Radio Frequency Design". If your skills are more at the technician level, you'll want the various ARRL publications.

Even if you're the egg-headedest egghead on earth, you'll want the ARRL handbook or one of their cookbooks, for the loads of circuits that actually work -- it provides a good starting point, or a fallback when some nifty idea you had fails to pan out.

--
Tim Wescott 
Wescott Design Services 
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Reply to
Tim Wescott

And make sure that your base drive can't get "stuck high" with the transistor being asked to deliver a 100% duty cycle! That would give you the DC equivalent of a near-dead-short to ground, limited only by the DC resistance of the choke and the Vce(sat) of the transistor.

Emission of some amount of Magic Blue Smoke is not unlikely.

Reply to
David Platt

Power? Small-signal? Wideband? Tuned? Impedances? Low noise?

--
John Larkin         Highland Technology, Inc 

jlarkin at highlandtechnology dot com 
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Reply to
John Larkin

The CE amp needs some impedance in series with the collector to produce gain, basically I to V conversion. Gain is proportional to the series impedance.

With a resistor, as you get more gain by increasing the resistance, the voltage at the collector drops. Basic Ohms law. But with an inductive load, you can have a large impedance at the operating frequency without the associate voltage drop of a resistor. You can get away with this because the RF amplifier is presumably narrow band.

Ideal inductors don't have thermal noise but ideal resistors do.

Either way, the amp is class A.

Reply to
miso

What the choke does is separate the functions of bias control from the RF energy flow. By putting a choke in series with the collector bias circuitry, the RF is loaded only by the output load. If the choke wasn't there the resistor that sets the collector bias current is also seeing the RF output voltage across it, and thus is uselessly dissipating RF power. See any of the data sheets on microwave MMIC amplifiers for further discussion. I think Minicircuits has some good app notes on the subject.

Reply to
Paul Probert

It doesn't need to be "narrowband." The choke simply needs to be high in i mpedance relative to the load. For example, if a conical inductor is used, then it can be somewhat widebanded, where the limitations are in the trans istor and other elements, but not so much the choke.

Reply to
Simon S Aysdie

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