Re: Okay, so, what am I missing here?

I don't know if there are "rules of thumb"... maybe just calculate rather than doing a NOLA white-trash guess ?:-) ...Jim Thompson

There's really no reason for that. The input impedance of a bipolar transistor amp could be less than the output impedance of the previous stage and you'd still get lots of gain. You'd be essentially multiplying betas in successive stages.

In a simple chain of common-emitter amp stages, all biased at the same current, without emitter degeneration, the 10:1 ratio isn't very reasonable.

Yup. And getting gain based on "multiplying betas" is an unpredictable way to "design".

I'd probably go with an MC1552-style feedback triple to get predictable voltage gain, but beef up the output to handle the speaker load... although why not just roll a little power amp from the get-go?

I don't think "Dave" could cope with the usual bi-directional intercom schemes. ...Jim Thompson

in=20

the=20

stage.=20

higher=20

Or=20

Well, i know of two ways to design multistage transistor amplifiers: =20 One starts at the beginning and starts with the required/desired input impedance. That sets up the bias network and then the rest falls out rather naturally. If the output voltage/current/impedance does not work yet add another stage. The second starts at the output requirements and proceeds to the input requirements. If there is noticeable excess gain in the system you can retune the = stages (with an eye toward reducing cost) or add global feedback. Global feedback after three stages is rather risky.

?-)

"Jim Thompson" wrote in message news: snipped-for-privacy@4ax.com...

Does it really make you feel good to talk that way about other people, calling someone you don't know a POS, and white-trash? Personally, I think it says a lot more about you than it does about me.

And no, I don't know anything about the calculations involved in anything as simple as an audio amp. Like I said, I'm making this up as I go along. I don't know anything. I'm just trying to learn.

Dave

He seems to be ripping on a lot of people lately - and that's about as close as he gets to posting on topic!

I tried to help, suggesting you calculate the bias currents. Rather than asking what I meant, if you're "just trying to learn", you smart-mouthed.

And I don't respond well to smart-mouthed brats... it's enough of a problem to deal with Larkin ;-)

HOWEVER... If you are really "just trying to learn", calculate the bias as I suggested... why is it so high? And how much gain do you really need? Put some numbers on things, and I'll try to point you in the right direction.

For really, I'll help... but take the chip off your shoulder. ...Jim Thompson

Here's an "intercom" I found at the first Google listing...

Maybe start with something simpler? ...Jim Thompson

You declared war on JL by being a spitefull old fart.

Take care now - you're way past fighting on 2 fronts.

Look at his posts. Nothing but boasting about how smart he is, how many things he's done, how many great ideas he has. But no content, no help, nothing but the occasional Spice graphs from who-knows-what secret circuits. He apparently expects all deference due to a self-declared "Master Circuit Designer" and gets all hen-squawkey when he doesn't get it. He doesn't seem to be interested in electronics at all.

He doesn't "try to help", he just tries to inflate his ego. It's not working. Sad excuse for "war."

So sayeth John "Dog Turd" Larkin ;-) ...Jim Thompson

Five ohm output impedance? Does that cause any reflections when doing TDR?

tm

Frankly, Jim, I think you've become so attuned to trading barbs with 'whoever' that the 'chip', so to speak, is on your shoulder and you misinterpreted his lack of knowledge as 'smart mouthed'.

He said from day one he knows next to nothing so when you toss out what are to you 'blindingly obvious' things like "bias current" he doesn't know what you mean because, as he explained, 'to him' bias is a voltage. That not being "smart mouthed," it's just all he knows from what he's managed to 'pick up' from somewhere.

I suspect you're used to 'challenging' EEs to 'think' but that isn't going to work here because he isn't an EE and will likely not understand what you're asking. You need to 'explain' things to him, not ask him to 'explain' it to you expecting that, in doing so, the 'light bulb' will go off and he'll 'realize' where you're going and the thing he's overlooked.

It's not for doing TDR. It's main use will be to drive amplifiers that in turn drive lasers or E/O modulators. I assume the thing I'm driving is a 50 ohm load. I could make it 50 ohms, but that would cut the max amplitude in half.

Next generation, I want to do at least 6 volts, truly 50 ohms... if I can find a driver circuit that can do it. Then I could drive E/O modulators directly.

He is!

That makes sense. I just wasn't sure you didn't have a typo in the specs. If there is a mismatch on the end of a cable, it could reflect back to the pulse generator and be re-reflected off the five ohm mismatch.

I like the packaging.

tm

The box is a standard Hammond enclosure. You see them (or possibly their clones) everywhere nowadays. Nice box, very EMI tight.

This is the PC board:

Frankly, the hardest part was getting the signals out of U8 (on the right) into the edge-launch SMA connectors, trying to keep a clean fast 50 ohm path. Rob and I did dueling EM simulations (ATLC under Linux, ATLC2 under Windows respectively) of the connector and PCB stackup, and it came out pretty good, some unknown mix of wisdom and dumb luck.

There is a small inductive glitch at the connector/PCB transition, as TDRd on the actual board. ATLC, being a 2D simulator, isn't up to stuff like this. I'm theorizing that's caused by a tiny gap between the connector PCB pads and the edge of the board. The fix would be to move the connector footprint about 20 mils to the right. Then the PC house would be cutting away copper when they route the board outline, which means they would probably call us and tell us that we did the layout wrong, and we'd have to tell them to go ahead and cut the copper.

Some day I should lay out a board with maybe a dozen edge-launch variations, just to find the best one.

Why not just notch the board 20 mils to move the connector in closer. You can then put a lock washer between the flange and the case. Might also solder the ground plane all the way across the connector in addition to the pins.

Doing a test board would be an interesting experiment.