Help with amplifier design (high quality)

[CrazyChipMan]

Hello !

I´m not new to electronics, but new to amplifier design.
This would be my first DIY Amplifier.

I studied lots of schematics (many of them found in this forum...excellent forum!) and tried to make my first design.

The aim is to develop an amplifier with high accuracy, output power is less important (about 50W shall be enough).

Before I start to calculate exact values and operating points I have some questions to you.



What you see is:

Input Stage: differential amplifier with reference current souce, output formed by current mirror.

"Voltage gain" stage: bootstap configuration for constant input impedance to the input stage, a bit feedback provided by R18

"current gain" stage: cascoded emitter follower to get a linear output characteristic curve.

output stage: A pair of complementary mosfets



Questions:

Is effective bootstapping possible over TWO stages (current gain and output stage) ?!?

Do the cascoded transistors (current gain stage) work with only a zener and a resistor at the basis? Zeners should be powerfull!

I want to drive the output mosfets without emitter resistors to get a high damping factor. For this, is it better to match them? Do you see any offset problems ?!?

I have NO Ideas for the semiconductors, everyone uses other ones and the market is very large and complex. IR HEXFETS for the output stage shall be a good choice ?!?

Thanx for any comment !!!

greets
Peter

[Tube_Dude]

Originally posted by CrazyChipMan


Is effective bootstapping possible over TWO stages (current gain and output stage) ?!?

Hi Peter

Yes, it is, as you have put in the schematic...over voltage gain stage and output stage.

Do the cascoded transistors (current gain stage) work with only a zener and a resistor at the basis? Zeners should be powerfull!

This cascoded transistor are in my view a very bad idea , because as the output mosfets don't have voltage gain , you loose in the cascoded stage , precious volts for full voltage swing.

I want to drive the output mosfets without emitter resistors to get a high damping factor. For this, is it better to match them?

Only if you use lateral mosfets, you can confidently dispense with source resistors , without fear of thermal runway .
In the case of vertical mosfets is not safe.

Use emitter degeneration resistors in the current mirror.

One last important point: why the inductor L1 at the input shorting the low frequencies to ground and inducing a large offset ?

Regards

[Netlist]

Peter,
I can't comment a lot as I'm not an EE but shouldn't the feedback R be R6?
Also, many R's and transistors have no value which could make commenting more difficult.
Last, in many schematics I've seen from Mosfet specialists like Nelson the value of the gate R's is a bit higher (roughly 100-400 ohm).
Looks nice otherwise.

/Hugo

[CrazyChipMan]

Hello!

Was a mistake with the L before the input stage, It should be a Resistor.
No values! because I have to choose the semiconductors first, waiting for your proposals....

@tube_dude
what are lateral mosfets ?

the voltage I loose at output voltage will be the voltage over the zeners, i suppose ?!?

greets
Peter

[The Saint]

Hi Peter

The way you have drawn the cascoded driver stage would only allow a voltage swing that is limited to the value of D1 and D2 minus 0.68v.

Get rid of the cascode and try a CFP driver stage.
or just connect the collectors of each driver to the supply rails.
Move C7 to Q16 across the Base, Emitter, better still use a 2 pole compensation network.

Insert degeneration resistors in the first stage current mirror start with 100 ohm and emitter resistor in Q1 and Q2 values starting at 10 ohms to 100 ohms each depending on the current set in differential stage.

R1 I would start with 330 Ohms and R2 22k Ohms this will set the current at about 2ma

Get rid of R18 and link it to the supply rail.

move your supply decoupling to in front of R17 and the bootstrapping does not have very good PSRR.
In fact I would use a voltage regulator there

source resistors in the o/p fets would be good. start with 0.22 to 0.47 ohms. 0.22 would work well.

You are also going to need a zoble network on the output to ground. start with 10 Ohm 5 Watt in series with 100nf 250v AC x2 Cap.

This should get you started and I think you will end up with a nice little amplifier.

[AKSA]

Anthony's comments are all on the money.......

However, another option to preserve your neat constant voltage driver.

Split the two Rs supplying the bases of the 'cascoding' transistors. Make them equal, and run two caps of 220uF from the midpoint back to the output rail.

This double bootstrap will enable you to swing the cascode bases well beyond the rail, giving very high rail efficiency despite using mosfets.

Cheers,

Hugh

[peranders]

Originally posted by CrazyChipMan
what are lateral mosfets ?

The current flows horisontally in the chip therefore a bit higher Rdson but on the other hand rather low capacitance's. Hitachi/Renesas make those. Lateral mosfets are very unsual these days. Now (sinces one decade back) you'll have vertical mosfets which means that the current flow vertically in the chip => much lower Rdson, much higher capactiance's also.

Rdson is an unimportant parameter as long as it's under 0.5 ohms or so.

[amplifierguru]

Hi Peranders,

....."Rdson is an unimportant parameter as long as it's under 0.5 ohms or so."

In a common source output amplifier, like my SKA, it's important as it determines the voltage gain in the output stage, very significant for preceding stage operational signal levels. Such amplifiers are coming to the fore as they make better, more efficient use of supply wattage, and circumvent the high Vgs losses of modern vertical mosfets.

Cheers,
Greg

[lineup]

Originally posted by The Saint
Hi Peter
Insert degeneration resistors in the first stage current mirror start with 100 ohm and emitter resistor in Q1 and Q2 values starting at 10 ohms to 100 ohms each depending on the current set in differential stage.

You can use 0.050-0.100 Volt as a target value,
for voltage aross these Q1, Q2 emitter resistors.
1 mA in each input transistor would make it 47-100 Ohm.

Same way you can put emitter resistors in Q5, Q6,
to improve precision in this current mirror.
Making voltage across these also 0.100 Volt or so.


I would also put supply rail filter ( R3 C1 ) at the other side of VAS. ( Between R10 R18 )
I think, but I'm not sure, same goes for R4 C4.

=================

What we all want to know, is of course, what is voltage supply of this amplifier.
What is your power output target value?
Will output run in Class AB, Class A?
Also, what output power transistors you will use?

This will effect how the rest of circuit should be.

HEXFET could be IRF540/9540 or IRF640/9640

[peranders]

Originally posted by amplifierguru
Hi Peranders,

....."Rdson is an unimportant parameter as long as it's under 0.5 ohms or so."

In a common source output amplifier, like my SKA, it's important as it determines the voltage gain in the output stage,..

Do we talk about the same parameter? Rdson is only valid when the transistor is saturated = not very important in a class A or AB amp. You may loose some power but in dB's its nothing.