Having set the required current, we measure the voltage between 0 V and the FET’s source.
We then apply +11 V to the drain of our Device Under Test (DUT), and using an ammeter adjust the programmable CCS to draw 80 mA through the DN2540N5. The programmable CCS probably needs a minimum of 6 V across it to operate correctly, so we connect it to a negative supply of perhaps −9 V. We know that when the EL84s are working, their cathodes will be at ≈11 V and will pass a total of 80 mA. It really is easier (and cheaper) to use the jig and fit a fixed resistor.Īs an example, we might want to use a DN2540N5 as a CCS shared by the cathodes of a pair of EL84s. Alternatively, a variable resistor could be used and Adjusted On Test (AOT), but we still need to know the approximate value required in order to buy the correct value of variable resistor and if accidentally set to 0 Ω at switch-on, the saturation current might cause damage. Unfortunately, the value of the source resistor has to be found empirically for each DN2540N5, using the test jig of Figure 2.58. The carbon gate-stopper resistor prevents Very High Frequency (VHF) oscillation, and the source resistor programmes the current. If there's nothing there, it doesn’t need to be filtered! This is so simply because there is so little high frequency energy developed in this circuit. It is particularly noteworthy that high frequency noise content (as opposed to the 40kHz fundamental related ripple) is unaffected by output filter characteristics. A two-section LC filter provides high ripple attenuation, although a single section will give good performance. The feedback resistor's ratio produces a 12V output. 4 Figure 8.5 is a 40kHz, 5V to 12V converter using the LT1533 in a push-pull, “forward” configuration. Operating the switching transistors in a local loop permits predictable, wide range control over a variety of situations. This ability to control the switching's rate-of-change makes low noise switching regulation practical. The voltage and current slew rates are independently settable by external programming resistors. The voltage across each transistor and the current through it are sensed and the loop control’s slew rate of each parameter. Each 1A power transistor operates inside a broadband control loop. The design's most significant aspect is the output stage.
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