How to match depletion mode mosfets

Rootz

Member
2010-01-23 5:03 pm
Dear all,

I'd like to construct mu and White followers with lnd150's. Problem is that the specs vary between components, so I need to match them. however, I'm not sure how. can anyone explain how and what to match for the intended purpose of the mosfets? Kind regards.
 

Rootz

Member
2010-01-23 5:03 pm
The lack of help made me do my own research better. Here's what I did. Use a fresh 9V battery to set the drain through a 100 ohm resistor at 9V, while keeping the source and gate at 0V. This gave me a first reading of the Ids at Vg=0 so I could match. Next I used a 220 ohm drain resistor and measured again. Surprise: the Vg=0 matched mosfets matched again (now at roughly -250mV gate). In all cases I measured voltage drop over the drain resistor, which will lead to some errors. My fluke won't measure very small currents, so this was the best I could do for now.

Will they match the same way with 250V? I don't know. Need to build a far better test jig for that. Saturation/performance curves in the data sheets look very flat all the way to 500V, but you know 'paper is patient'.
 
You probably want to match for an "outcome" -- let's say 4 mA. You can use a potentiometer as Rset between source and ground, and perhaps a 150 ohm gatestopper between gate and ground. Connect the 9V to the drain and adjust the potentiometer until you get 4mA flowing through the circuit.

LND150 doesn't have a large SOA -- safe operating area -- i would use a DN2540,

Walt Jung has his "current sources' atricles from AX archived on his blog/website.
 

Rootz

Member
2010-01-23 5:03 pm
Take a look at the application I use the LND150 in. For more current there are indeed better choices I admit. But I don't expect the follower to drive much current at all. It will go to a delay effect with a 1 meg ohm input impedance. Guitar effects (stomp boxes that is) tend to be high input impedance.

A follower with a LND150 will drive a 10k load to 2V p-p just fine. Not for Hi-Fi of course, but we are talking guitar effects loop here.

I had to build the loop in a small Marshall amp, so it needed to be small in size (max 50x70 mm). No room for TO-220, current sources, mu-followers... Such a pity :D
 

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