Transistor selection - Page 1
Forum section: electronics / projects

author: IconicPCB
author_rank: Super Contributor
timestamp: November 16, 2015, 01:26:42 am
content: Looking for an old fashioned IF AGC transistor in SMD package.This IF AGC technology is obsolete. Devices I could find were in TO-92 packages. Any hint of an SMD device welcome.

author: moffy
author_rank: Super Contributor
timestamp: November 16, 2015, 02:01:03 am
content: The BF998 works well:http://nt7s.com/2008/07/dual-gate-mosfet-investigations-gain-and-agc/

author: IconicPCB
author_rank: Super Contributor
timestamp: November 16, 2015, 03:25:28 am
content: Moffy,Thanks for the input.It must be a BJT as I only have 5V supply to play with.

author: rs20
author_rank: Super Contributor
timestamp: November 16, 2015, 03:32:46 am
content: Maybe there's context here I'm missing, but MOSFETs run just fine on 5V or less...

author: dacman
author_rank: Frequent Contributor
timestamp: November 16, 2015, 03:38:52 am
content: Would this work?http://www.nteinc.com/specs/300to399/pdf/nte313.pdf

author: IconicPCB
author_rank: Super Contributor
timestamp: November 16, 2015, 07:48:05 am
content: RS20,BF998 is a dual gate mosfet. As such it operates in a cascode configuration such that there has to be at least a few volts difference between the two gates. At the same time the lower gate needs to be a few volts aboveground/source potential so that the overall device needs to operate from anything say tenish to fifteenish rails.Dacman, the NTE part number looks promising. thanks. Reason for SMD requirement ..machine assembly

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 16, 2015, 03:53:55 pm
content: How is an "IF AGC transistor" different from any other conventional type?If you can't provide specifics, I say just throw a2NMMBT3904 at it...Tim

author: Ian.M
author_rank: Super Contributor
timestamp: November 16, 2015, 04:16:24 pm
content: Its a *LONG* lime since I studied that and it was only mentioned in passing, but IIRC it needs to have a large and reasonably linear change of Hfe with Ic so its gain can be controlled by varying the bias point. The MMBT2904's gain curve is essentially flat between 0.1mA and 20mA so it would be a poor choice for this application. Also, you'd probably need to specify a manufacturer as the shape of the gain vs Ic curve may be rather different for a second source part NOT designed for forward AGC operation.

author: IconicPCB
author_rank: Super Contributor
timestamp: November 16, 2015, 09:11:48 pm
content: Thank You Ian,Tesla to illustrate the point here are two device data sheets.compare and contrast the plots of Beta versus Ic and gain bandwidth product versus IC plots.Fairchild S9018 was specifically designed to have a "poor" gain bandwidth spec.It was used in IF AGC applications such that the bias point would be altered depending on detected IF level.Naturally this utility is present nowadays within various highly integrated IF strips.All i need is an SMD device with 9018 like performance.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 16, 2015, 09:20:25 pm
content: MMBT3563 (if it exists..?) or MMBTH10 might be closer, considering the lower Ccb and higher fT.Tim

author: dacman
author_rank: Frequent Contributor
timestamp: November 16, 2015, 10:47:45 pm
content: MMBT2222There are several variations at Newark.http://www.farnell.com/datasheets/1644026.pdf

author: rfeecs
author_rank: Frequent Contributor
timestamp: November 16, 2015, 11:50:17 pm
content: Quote from: Ian.M on November 16, 2015, 04:16:24 pmThis doesn't sound quite right. Hfe is the current gain, and it is typically fairly constant over a wide range of collector current.On the other hand, the transconductance gm is roughly proportional to collector current. This is true of every bipolar.Gain is mainly determined by gm, not Hfe.So just select a part with similar beta and Ft.

author: moffy
author_rank: Super Contributor
timestamp: November 17, 2015, 03:32:28 am
content: I am pretty sure you could get the BF998 to work from 5v. It has a 50db change in output power with a 1v variation on the AGC from 2v to 3v. Just depends on your signal level as to whether you have enough headroom left.