Lm4702 Bpa

[gmikol]

Okay...I'll admit, I'm pretty green...but I can't understand how this output stage works. The other thing is that it looks like the base resistors on the low-side drivers are shorted to the output. That can't be right, can it?

Any help to the un-informed is appreciated.

--Greg

[halo0925]

The output stage uses a common-emitter stage for the negative current gain and a common-collector stage for the positive current gain. This type of output topology is commonly called a Quasi output in which all transistors are the NPN type.
The source and sink from the lm4702 provide the drive for Q8 and Q9.
While R22, VR1, R23, and Q1 provide thermal managment. C16 and C17 along with C10/(compensation) provide frequency and stability control.
Q8 and Q9 provide current gain for Q2/4/6, Q8 is set up as a common collector while Q9 is a common emitter. The emitter of Q8 drives the base resistors R26-28 and in turn Q2/4/6. While the collector of Q9 drives the base resistors R29-31 and inturn Q3/5/7.

[gmikol]

Still don't get it...feeling kinda stupid tonight. I don't want to derail your thread, but you got a link that explains the Quasi output stage?

Thanks much--

Greg

[space]

I'm to be considered a noob when it comes to this, but if I read the datasheet for the 4702 correctly, it can only source/sink 5mA typ. If this is the case, will your circuit have enough gain to allow for the output Imax that it might require? (What are Q8/Q11?)

My "noob" calc:

Imax=140V/4 ohm=35A (worst-case?) (this is HI power)
= ~12A per MJ18020
with a gain at about 10 (worst-case)
=1,2*3=~3,5A total base drive current
Q8/Q11 (Darlington’s?) then needs to have a gain of at least
1A/5mA=700 at 1amp

As stated earlier, consider this a noobs post, also checking if he is understanding the calculations needed to check the output drive capability. What he do remember though is many of the other threads of HI-power amps where the current of the drivers often are mentioned.


space

[halo0925]

Space, Thank you, the first imformed answere.
Yes I do know the math and I am looking for suggestions on things just like this, Q8 and11 are representative at this point, I had played with another current gain stage or a darlington, I dont like either option, looking for a NPN high HFE with enough push any suggetions?

P.S. there are transistors out there that are not darligntons that offer enough current gain, but I would like to keep the design within affordability.

[bushroot]

Newb question...could you use an IGBT?

[consort_ee_um]

I do not think this circuit will work. Your output transistors connected to the negative rail need source current and you have them tied to the sink output of the chip

[jackinnj]

the problem with Quasi complimentary, at least as Mr. Self describes it, is "poor symmetry around the crossover region" -- which can be bettered with the use of a Baxandall diode.

[dfdye]

I am confused as to the whole premise behind this design. Are you trying to parallel the two channels of the 4702? That doesn't really make much sense to me since you can accomplish the gain benefits by modifying the output stage. It would seem that the trouble of balancing the two inputs coupled with the fact that you would still only get ~10mA out of the two sides of the chip, and that it would probably have a pretty big impact on the stability of the circuit. . . . .

Anyway, I guess I am saying that I would handle the additional power on the output of the amp, and not worry about paralleling the 4702. If you just want one channel out of the amp, just use half of the chip. Plus this gets rid of the input buffers and you can bring the signal straight into the chip. The way you have the circuit, it looks as though you are building an amp with two output stages for the same signal. Is this right, or have I been smoking too much crack?

As for the question regarding Darlington alternatives: Typical power transistors have a gain of ~30 max on a good day. The only way you will get additional gain is to use another transistor before these devices, either on a different package, or within the same package (aka Darlington). Regardless of what you call it, you will have to use more than one transistor if you want any moderate level of power out of this chip. 5mA is obviously not near enough to drive a speaker properly, nor is 150mA, so there needs to be an additional current gain device between the chip and the output. I am not sure your objections to Darlingtons, so if you could be a little more specific as to what you are trying to get/avoid, I think we could offer a little more help.

[glennb]

Originally posted by consort_ee_um
I do not think this circuit will work. Your output transistors connected to the negative rail need source current and you have them tied to the sink output of the chip

You are correct. It will not work. An output stage like Jackinnj's post (with the baxandall diode) needs to be used, so that the Vsource & Vsink pins on the LM4702 will have a low ~constant voltage bias between them.

The U1-3 arrangement seems to be overly complex to just provide a phase splitting funtion. It is possible to configure the LM4702 halves as inverting & non-inverting with the same gain and opposite phase and dispense with U1-3.