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Class B distortion

Started by Unknown December 20, 2014
meow2222@care2.com wrote:

> Now, the nfb is taken from the circuit's output terminal, not from the > opamp's output,
Does this sentence make sense? The output stage is the output of the opamp. In any event, you can use any number of ways to make the circuit AB: 1) VBE multiplier 2) diode string 3) Complementary class A buffers to eliminate the drop I'm trying to think of any integrated class B amps. Maybe motor drivers where you wouldn't care about the notch.
miso <miso@sushi.com> Wrote in message:
> meow2222@care2.com wrote: > >> Now, the nfb is taken from the circuit's output terminal, not from the >> opamp's output, > > Does this sentence make sense? The output stage is the output of the opamp. > > In any event, you can use any number of ways to make the circuit AB: > 1) VBE multiplier > 2) diode string > 3) Complementary class A buffers to eliminate the drop > > I'm trying to think of any integrated class B amps. Maybe motor drivers > where you wouldn't care about the notch. > >
LM386? -- ----Android NewsGroup Reader---- http://usenet.sinaapp.com/
On 2014-12-21, meow2222@care2.com <meow2222@care2.com> wrote:
> On Saturday, December 20, 2014 12:11:02 PM UTC, Phil Hobbs wrote: >> Yup. And to get anywhere near it, they have to run a huge input error voltage--as much as a volt for FET types or bipolars with lots of input stage degeneration. >> >> That's not a huge problem when the amp itself is reasonably linear, but it sur is in this case. >> >> Cheers >> >> Phil Hobbs > > Does that mean if I ran the opamp at gain=1 it would at least reduce the problem to some degree?
yes, or if you reduced the frequency, or input amplitude. anything that makes the curve flatter helps against slew-rate limitations. -- umop apisdn
On Sun, 21 Dec 2014 10:34:21 +1000, <meow2222@care2.com> wrote:

> On Saturday, December 20, 2014 8:57:22 PM UTC, Jan Panteltje wrote: > >> + >> | >> |/ c >> -------| NPN >> | |\ e >> in ---- +|\ | R | >> | >------===----| >> -- - |/ | |-------- out >> | | |/ e | >> | --------| PNP | >> | |\ c | >> | | | >> | - | >> -------------------------- >> >> For some value of R the cross over is eliminated as the opamp drives >> the load for small signals. > > Yes, I tried that in spice. It seems the R has to be very low for an > opamp though. 0.65v on 8ohm load = 75mA. 75mA 0.6v = 8ohms for the R... > plus 8R load makes a 16R load. Can a random opamp drive that? > > > NT
Jan's circuit works. R is actually there for stability not to drive the speakers (check it out 20ma output current into 8 ohms - a whopping 3.2mW, into a standard speaker is below the noise floor in most indoor locations). An R about 4.7k +/- 50% should be fine. If you already tried this and crossover was really bad there must have been something very wrong in your circuit. Even with a 741 the cross over distortion is above 20k.
On Sunday, December 21, 2014 6:02:25 AM UTC, bitrex wrote:
> miso <miso@sushi.com> Wrote in message: > > meow2222@care2.com wrote:
> > In any event, you can use any number of ways to make the circuit AB: > > 1) VBE multiplier > > 2) diode string > > 3) Complementary class A buffers to eliminate the drop > > > > I'm trying to think of any integrated class B amps. Maybe motor drivers > > where you wouldn't care about the notch.
> LM386?
Looks like AB: http://i.stack.imgur.com/rHBqT.png NT
On Sunday, December 21, 2014 5:15:15 AM UTC, Tom Swift wrote:
> meow2222@care2.com wrote: > > > On Saturday, December 20, 2014 2:38:14 PM UTC, Tom Swift wrote: > > >> Add a complimentary NPN/PNP emitter follower stage between the op amp > >> and output to bias the output stage so it is barely on, and to add > >> more drive > > > > I'm not managing to work out what you mean there > > Take 2N2222 and 2N2907. Tie the bases together to the output of the op > amp. > > Connect the collectors to the appropriate supply voltage. Works best with > symmetrical supplies but can be made to work with a single supply if you > keep it centered. > > Add 1k resistors from the emitters to the appropriate supply voltages. > You now have two emitter followers with 1.2V between the emitters and > centered on the output of the op amp. > > Tie the emitters to the base of the opposite polarity output transistors. > > They each now have 0.6V bias to turn them on. > > Add 22 ohm resistors to the output emitters to stabilize the current. > > The junction of the 22 ohm resistors is the output signal. Tie this to > the load and to the feedback for the op amp. > > This would be so much simpler in LTspice but I got to go and finish my > laundry.
I think I follow. I can see possible issues, will spice it later and see NT
On Sunday, December 21, 2014 5:14:14 AM UTC, bitrex wrote:
> meow2222@care2.com Wrote in message: > > On Sunday, December 21, 2014 3:30:45 AM UTC, bitrex wrote: > > > >> Could you use a large-ish resistor in the op amp power supply > >> lead, with a small current sense emitter resistor on the power > >> transistor and another transistor connected to pull base current? > >> Bypass the emitter resistor with a capacitor. Then I think it > >> would self-regulate and you wouldn't need to tailor the opamp > >> sense resistor so much. > > > > That sounds like a current limit. Then if the opamp lead R is too high, the op trs will sit drawing lots of current, just being limited to i_max. It doesnt sound good. > > My idea was that it's not a lot of current - just enough to keep > the power transistors conducting a little so their bases are > sitting at ~ 0.6 volts without having to match the opamp resistor > to each op amp...you'd just need to select it for the minimum > expected quiescent op amp current. If the current was higher the > second transistor would pull away the excess. The capacitors > bypass the emitter resistor on the power transistor for AC; the > emitter resistors are just to set the DC operating > point. > > Maybe this won't work and I'm missing something?
If you add a C so it limits mean current, that's still going to be much too high for idling. NT
> > If one left out the 2nd tr that steals base current from the main one, I think I'll sim it, see if it does what I think, sounds interesting.
On Sunday, December 21, 2014 10:08:00 AM UTC, David Eather wrote:
> On Sun, 21 Dec 2014 10:34:21 +1000, <meow2222@care2.com> wrote: > > > On Saturday, December 20, 2014 8:57:22 PM UTC, Jan Panteltje wrote: > > > >> + > >> | > >> |/ c > >> -------| NPN > >> | |\ e > >> in ---- +|\ | R | > >> | >------===----| > >> -- - |/ | |-------- out > >> | | |/ e | > >> | --------| PNP | > >> | |\ c | > >> | | | > >> | - | > >> -------------------------- > >> > >> For some value of R the cross over is eliminated as the opamp drives > >> the load for small signals. > > > > Yes, I tried that in spice. It seems the R has to be very low for an > > opamp though. 0.65v on 8ohm load = 75mA. 75mA 0.6v = 8ohms for the R... > > plus 8R load makes a 16R load. Can a random opamp drive that? > > > > > > NT > > Jan's circuit works. R is actually there for stability not to drive the > speakers (check it out 20ma output current into 8 ohms - a whopping 3.2mW, > into a standard speaker is below the noise floor in most indoor > locations). An R about 4.7k +/- 50% should be fine. > > If you already tried this and crossover was really bad there must have > been something very wrong in your circuit. Even with a 741 the cross over > distortion is above 20k.
I did sim it. Putting that R in made little difference, still heavy crossover distortion. Maybe I need to go much lower with R. How well your average opamp can cope with very low R I'm not so sure, lets hope Spice models it ok. NT
On Sunday, December 21, 2014 5:50:57 AM UTC, miso wrote:
> meow2222@care2.com wrote: > > > Now, the nfb is taken from the circuit's output terminal, not from the > > opamp's output, > > Does this sentence make sense? The output stage is the output of the opamp.
opamp's output goes to op tr bases. Op tr emitters are the speaker output terminal. If you connnected those 2 points there would be no op tr drive.
> In any event, you can use any number of ways to make the circuit AB: > 1) VBE multiplier > 2) diode string > 3) Complementary class A buffers to eliminate the drop
So far I've run into issues with those 3. 1: hot tr Vbe=0.35v, vbe multiplier isnt self adjusting and theres no feedback to deal with excessive idle current 2: ditto 3: will sim this, though I suspect issues there too
> I'm trying to think of any integrated class B amps. Maybe motor drivers > where you wouldn't care about the notch.
NT
meow2222@care2.com wrote:

> I think I follow. I can see possible issues, will spice it later and > see
Post your LTspice file so we can check it.
> NT