Yes, I did view the current through R12 which looks normal. The thing operates class AB, so only one transistor is on at a time, so a 50% discontinuous current is normal. I did improve the distortion using a
1458 op-amp in place of the 358. Looks much better now. The problem now is I only get 1.5 volts peak into 8 ohms with an 8 volt supply and I was trying for 3 volts or more. The HFE figure for the 2N2219A is minimum 40 at 500mA or 7.5mA at 300mA. The 120 ohm resistor draws .
7/120 =3D about 6 mA so the op-amp must deliver 13.5 mA and the spec sheet says only 10 mA short circuit. So, apparently, it needs higher gain transistors or an op-amp with lower output impedance, or both. Any ideas?
Your best bet would be to take advantage of your ISP service, that is, if you're using a private one instead of those free Email or economical services.
I know times are tough and all the starving chiefs out there are trying their best to profit off the innocent indians. We wouldn't want them to be reminded of what it is like to actually to do something for that money.
As far as image hosting sites, I am sure you can find another that isn't so overwhelmed with useless garbage.
Your provider should be giving you a small chunk of personal web space. It's an easy thing to do. You simply navigate your browser using the copy and paste to your page or use a FTP program. Then you provide the link to your site plus the name of the image you put there.
By the looks of your address, it looks like you should be able to get one that provides you a clean page, not one that is a child form of a business page, which is just as bad!
Myself, I have the option of 5 E-mails, each one around 100megs of space the last time I checked. It's been a while.
That op-amp does not pull the reals, the 358 will do that effect on the load side and there by give you more v to bias the transistors.
I don't think you have a current demand problem, you may have a rail to rail problem how ever.
The 1458, as old as it is, still has a lot of usages. The las time I looked, that op-amp (dual) only provides ~ Vcc-1.5 and Vee-1.5. Here you have lost 3 volts to start with. This now gives you 5 volts to play with.
Of course, you really don't want to saturate the amp, so lets assume you have only 4.5V to work with..
split that in half, since you looks apparrent you are operating in Class A state on the output side of the op-amp and you get ~ 2.25 volts Peak to play with.
Now., let us not forget, the minimum required for each of those transistors to start working. ~ 0.7 and then times this by 2 and you get
1.4. Remove that value off the top and you are now getting closer to where the problem is.
That configuration you're using in the first place is fighting against you. As one side is conducting the other side is still conducting, just about all the way through. This is going to remove a good chunk of your output.
Have you considered a config like the following or something in this line?
What sets the quiescent current thru the PNP-NPN path? ...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
A circuit image should compress extremely well because it contains pixels of a few colors, and most of them are white. Therefore you should be able to post it right to the newsgroup, uuencoded into a single article.
(Yes, I can hear it now: ``gasp, a binary to a discussion group?'')
Well, if it is small and on-topic, what is the harm?
The benefit is that is that the picture permanently stays with the discussion.
These URL's people post will be long gone, making much of the archived discussion junk.
In Ltspice it was ~ 70 ma with that basic circuit in both the PNP and NPN. I was using 700..800 ma trannies.
Putting an R between Vcc and B of the PNP will drop that down a bit and bring it closer to being symmetrical. Also, you can use a R on the op output to the collector bridge to lower the Quies.
I Didn't save the spice I used but I am sure I can put it together again and post it if you wish, I just used what was in the stock lib.
Try it in the real world. Have lots of spare transistors on hand :-) ...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
Actually Jim, I have done that in real world cases. Which is why I suggested it.. It does work when you use the correct set of components and is a basic of many designs that i've seen over the years. A very particle way to having current outputs
Old time 741 with Emitter outputs and related type power amp Ic's work very nicely with a config like that. Of course in many cases, you need to tailor the bias a little with some bypass R's on the Rail to the base of each side if the Qu current is too high.
I had a load of LM380's years ago that I put to some good use making little half bridge servos with a circuit of that type.
The only problem with that design is, if the driving op-amp happens to circuit for some reason. It will take out the outputs. A current limiting R on the op-amp output to the collector node is a good practice, something I did not use here in this example. And don't forget the bypass bias R's incase the QU Is too high in the chip that is used.
I know your an old pro and I am sure you have had your share of smoking some silicon. :)
Larkin will bless your work. Me? I do not recommend it... do yourself a favor and do some Algebra. On a job interview I'd toss your ass in a blink ;-) ...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
That's weird looking too! There's a JL circuit with tran's in the power leads, but I think it's push-pull. You can get a lot closer to the supply rail that way... At least that's my understanding. I've yet to try a circuit with power gain in the supply leads.
Why would the op-amp rail to rail limit be a consideration? Imagine the transistors have a large HFE of say 1000. There will be a large voltage gain from the op-amp output to the main output. If the main output is moving rail to rail, the op-amp output will be moving much less, maybe only 1 volt or so.
I did that sort of thing as an LDO, ~1970... it's in a hybrid I made for the TOW missile. You need base-emitter resistors such that the "crowbar" current is zero. Disconnect OpAmp output from as shown, then connect it via a resistive divider between rails (it'd be a single resistor to ground if split supplies were used. It's then a CLASS-B amplifier. ...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
That is only a rough lay out, to lower the quiescent current you put by pass R's on the bases of the transistors from the rails. You need to know the quiescent current of the op-amp you're going to use as part of the equation, I used Lpspice on that one and picked a single rail op-amp out of the list.
OK. Show me your math. What's the quiescent current thru the PNP-NPN path?
Those Youtube videos Bitrex found of IIT lectures are based on chips I designed 50 years ago... all math... no simulator existed until about
25 years into my career. Also have some of my chips used as class examples at Rochester Institute of Technology.
Calculate that quiescent current and get back to me with the result. (Show all your work :-) ...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
Which at least I can understand. The transistors turn on at ~ 1 amp. (Seems like a lot for an opamp...) But that's easy to scale down. (I'd only want an amp total so a 30mA from the opamp should be fine.)
I like that the opamp 'handles' the cross over distortion.
The LM1458 ( a dual 741 really) is not specified for supply rails as low as
+/- 4 volts - idle current draw will be low, maybe under 1 mA for both amplifiers.
The NPN and PNP transistor types are not specified so may have Hfes of anywhere from 30 to 300 or more - taking the middle ground in both cases, idle current is gonna be around 100mA. But the range is anywhere from 20mA to 500mA.
But the big one is how that heck does the schem shown cope with differing Hfes for the two transistors ?
The usual topology has the op-amp delivering current into the common or ground via a low value resistor and feedback taken only from the two collectors. This way, the op-amp can have differing rail currents at idle.
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