I'm looking at this schematic and am wondering why the designer chose a
2N2369 over a 2N2222 or 2N3904? I looked at the datasheet for the 2N2369, and I didn't see anythng amazing about jumping off the page. Did I miss something. The maximum frequency expected would be under 30MHz.
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He also makes a big deal out of the particular op-amp chosen as (what appears to be) a simple voltage follower. Any reason that you all can think of?
Ok then, I was just wondering if there was something special about it. It does specify a much faster Toff, but that is with a much lighter collector current (if that matters?).
OK, well that's not a big problem either. I notice he only wants a range of
2N2369 is GOLD-DOPED, thus fast recovery from saturation (if over-driven).
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
America: Land of the Free, Because of the Brave
Like this Eeyore.....this is the kind of thing I was hoping to see......an actual reason to use that part over a more common jelly-bean. I'm guessing this is where the Toff thingy comes in? Of course it's now a moot point since it's an obsolete part, but maybe the designer liked the metal cans.
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
America: Land of the Free, Because of the Brave
Is Toff the thing I should have been looking at to determine this? Gone are the days of loads of charts relating everything, now I mostly see a few paramaters specified that (at least AFAIC) leave no way to extrapolate anything useful. :-?
Yes. Though I've only "looked" at a 2N2369 in the past 20 years ;-)
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
America: Land of the Free, Because of the Brave
If I had to choose between the 2N2369 and the 2N2222 (for your application), I would choose the first one.
The 2N2222 is designed for higher current operation (larger die size, higher capacitance). Because of higher Ft and lower reverse capacitance (2n2369 device), the gain of your circuit will be higher, so you need to pick up less signal from your local oscillator. Check the datasheet from Fairchild MMBT2369A (same chip in different package).
At low current (for example 3 mA), the Ft of the 2N2222 device drops to below 200 MHz. It remains high for the 2N2369 device (they specify at 500MHz at 10 mA). When you overdrive (as Jim mentioned), the 2n2369 device remains fast, so you get good pulses out of it. The 2N2222 device will perform less (at your relative low currents).
The 2n3904 is in-between the two.
Regarding the opamp, I do not have the datasheet, maybe somebody knows whether it is equal to the OP-07.
When using the 2369 or similar gold-doped transistors with fast switching times just keep in mind that their hfe is much lower than what you may be used to from the 3904. That's the price to pay for speed. IOW there is no free lunch:
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BTW the TO-92 version of this one seems to be on the way to memory lane, could become hardcore unobtanium soon. Unless Jim can spare some from his bag.
The author might have used it in the circuit you posted because he needed the speed but not much in terms of current gain. Just one comment here: It isn't a good practice to drive a logic gate straight off the collector of a transistor. There should be a fast Schmitt first. If the input signal is smallish this counter might otherwise display next week's lottery numbers instead of the frequency.
That'd be my guess too. Most of us design with whatever we're comfortable with, & have in the parts drawers. In my case, I use a lot of BC5xx transistors because they're what I grew up up with, & I can buy them in bulk locally at dirt cheap prices.
When you're working from a circuit like that one, as long as you've checked the spec's of the original part, there's no harm in substituting something equivalent that you already have in stock.
--
W "Some people are alive only because it is illegal to kill them."
. | ,. w ,
\|/ \|/ Perna condita delenda est
---^----^---------------------------------------------------------------
I wonder if the modern versions of the 2n2369 and similar parts are still gold doped. I have about 33 datasheets of various versions of '2369 transistors stored in my computer, and only the very oldest ones, like Raytheon's datasheet for the bare die, mention the gold doping.
I would presume they are still gold-doped. Otherwise how would they meet the storage time spec?
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
America: Land of the Free, Because of the Brave
How are RF transistors made, anyway? Small-ass dies, dash of spices (like gold) to reduce recombination, etc.? Is it then impossible (with silicon) to maximize all parameters? Like, say I want a transistor that does like...
600Vceo, Ic > 1A, t_stg < 10ns, and hFE > 200. (Nevermind lead inductance on actually putting that much collector current and bandwidth into practice...)
Tim
-- "Librarians are hiding something." - Steven Colbert Website @
There will be trade-offs. Such as the hfe penalty that usually comes with gold-doping. Don't expect much above 50, typically a lot less.
Then there are market forces at work. There isn't a lot of market for high voltage RF transistors. Some of the power horses I grew up with have vanished :-(
Yeah. I mean, HV transistors usually have appalling hFE anyway (typical line output transistor circa Vceo ~ 1.5kV, hFE = 4, t_stg = 3us!), but that's optimized for something else. What's the ultimate maximum between Vceo, hFE, fT, etc. that can be physically produced, you know?
I certainly wouldn't expect to find something like this for sale! Most RF amps are modules at what, 48V or something? Throw together a thousand (plus a nutsoid transformer and rectifier for the kiloamp supply rail) and you've got a massively-redundant FM transmitter...or something like that...
Tim
-- "Librarians are hiding something." - Steven Colbert Website @
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