A look into National Semiconductor’s LM4702 VAS

First thing here for those that aren’t aware:  “SANTA CLARA, Calif., Sept. 23, 2011 /PRNewswire/ — Texas Instruments Incorporated (TI) (NYSE: TXN) today announced the acquisition of National Semiconductor (NYSE: NSM) is complete.”

I thought the first paragraph is relevant to our subject based on the confusion come across in forums on who owns what.

The LM4702 driver

Texas Instrument’s  Application Report SNAA031A–May 2006–Revised May 2013 covers the AN-1490 LM4702 Power Amplifier.

The champions of the Forums Brains Trust and slew of experts world wide will always make some comparison statements to the effect that National is better than Sanken or Vice Versa.  Of course, I am not that clever except to note that both Sanken and National, now TI, are powerhouses when it comes to chip manufacture and I doubt they hired a bunch of idiots for their design room.

Sanken cloak and daggers stuff

Sanken chips are used often in Sony amplifiers, especially in their VAS (Voltage Amplifier Stage) whilst in my own research one is of the opinion (mine of course) that the LM4702 series has opened up the market for chip amp designers and experimentation thanks to their application design notes. Not so with the Sanken or NEC chips where datasheets are as scarce as rockin’ horse shit!

Part of this article was driven by my own interest in Sony amplifiers and the accessibility of their top class schematics available legally or illegally on the net. The issue with the Sanken chips is availability of suitable literature covering their application and design notes.

STK350-630TS-E-UPC2581 and LM7402

The three chips shown above shows the similarity between the Sanken STK350-630, the NEC Regasys UPC2581 and the TI/NS LM4702.  Be aware though that doing repairs on amplifiers using these VAS drivers that sometimes things aren’t as simple as they seem – hence the need for a schematic.  See eservice.com for some light on this subject.

Renesas UPC2581

According to reader’s comments it appears that the STK chip was out for some considerable time before the release of National Semiconductor’s competitively priced LM7402.  In my own experience I found both products behaved in a similar fashion their design lent themselves to both Darlington BJTs and MOS devices with low switch on voltages.

LM4702A high-fidelity audio power amplifier driver

LM4702A amplifier project.

The bias stage:

When building any amplifier which uses a VAS chip to drive the output stage one must not forget a very important aspect to the design – the Vbe bias of the BJT stage.

Biasing power Darlingtons

Setup variable bias for NPN/PNP Darlington’s using the LM4702

Using the ever popular STK350-630 and MN2488 / MP1620 configuration in Sony amplifiers