Switches controlled by the input signal amplitude are used as input signal level indicators, input signal level analyzers, power supply voltage indicators, and so on. Such switches are usually made as a line of comparators or bipolar or field-effect transistors in combination with an input resistive voltage divider.
The switch described below is made using MOSFET Q1–Q11 2N7000 and is characterized by a fairly simple scheme, the ability to increase the number of switching channels, as well as the presence of non-overlapping switching windows. This feature allows you to use a multi-position switch controlled by amplitude as a multi-load switching device with control from multiple equivalent control panels using just through two wires.
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These panels contain several switches and resistors that are connected in series with them, and the unused pins of the switches are connected together a nd connected via the control line to the device’s power bus, and the unused pins of the resistors are also connected together and connected via the control line to the Multi-position switch input.
The multi-position switch, Figure 1, works as follows. The input signal is fed to the resistive divider R1–R6. From resistive divider R1–R6 through resistors R7–R12 control signals are sent to the gates of field-effect transistors that control loads R13–R18. In parallel to the control circuits (source-gate) of transistors, transistors are connected that provide sequential shunting of these circuits as the input voltage increases.
Figure 1 Electrical diagram of a six-position switch controlled by applying different levels of voltage to its input.
In the initial state, if there is no control voltage, a high-level voltage is present at all six outputs of the device, almost equal to the power supply voltage of the multi-position switch (from 6 to 60 V).
For the first of the channels, the output voltage drops almost to zero, Figure 2, when a voltage from 2.4 to 4.1 V is applied to the input of the switch.
Figure 2 Dynamics of electrical processes (switching output stages) of a multi-position switch when a linearly increasing voltage is applied to its input.
For the second channel, the switch state switching window is in the range from 4.9 to 8.0 V. for the third channel – from 9.5 to 12.8 V; for the fourth – from 15.2 to 21.6 V; for the fifth – from 25.3 to 32.7 V and for the sixth-over 38 V.
The maximum input voltage of a multi-position switch no more than 40 V. Under this condition, the maximum possible voltage on the gates of transistors does not exceed their maximum permissible passport values (no more than 20 V).
If desired, the voltage switching ranges of channels can be adjusted by selecting the values of the input resistive divider R1–R6.
According to the 2N7000 datasheet (ON Semi), gate threshold voltage may vary from 0.8 to 3V = 375%. How is this fact compatible with your design’s apparent claim that a ~20% change in Uin can switch between one “position” and another?
The threshold switching voltage of a MOSFET is the voltage at which a conducting channel opens and current begins to flow through it. For the 2N7000 transistor, according to the technical description, the gate threshold voltage may be in the range from 0.6 to 3 V, but in fact it is 2.0-2.1 V. See also the Technical Description, FIGURE 2-1: if the voltage between the gate terminals and the source of the 2N7000 transistor is less than 2.1 V, the transistor will be closed.
Sorry, Michael but that’s not correct. The 2N7000 datasheet (if you’d bothered to read and understand it) defines gate threshold voltage as the Vgs voltage that will produce a drain current of 1mA when Vds = Vgs, and specifies that this voltage can be anywhere between 0.8 and 3V.
Short version: Your design idea won’t work with real transistors. It’s seems unlikely you ever built and tested it, or you’d know this.
Dear Stephen, thank You for Your comment. You are absolutely right, with Vds = Vgs, the voltage Vgs, which will generate a drain current of 1 mA, can range from 0.8 to 3 V (a typical value of 2.1 V). However, see the Fairchild technical description, https://pl-1.org/getproductfile.axd?id=7215&filename=2N7000.pdf&ysclid=l2b7ccbtdk , with a drain current of 0.25 mA, this voltage is already in the range from 1 to 2.5 V, and the typical is 2.1 V. If You noticed, a 3 MOhm resistor is included in the load circuit of the 2N7000 transistor, the drain current is limited to a maximum value of 13.3 µA at Vds < 40 V, and the range of Vgs values narrows to 2.0-2.1 V.
The spec’ you quote is for the 2N7002, not the 2N7000. Furthermore lowering drain current from 250uA to 13.3uA would LOWER the minimum threshold Vgs below 1V, not raise it to 2V.
PS: I suggest you consider using an actual voltage comparator like the 339 for your switching elements. It, unlike the 2N7000, was designed for — and would therefore likely actually work in — such an application.
https://www.ti.com/lit/ds/symlink/lm339.pdf?ts=1650679135441
Dear Stephen, thank You again for Your comment. You are right, earlier in the comments I mistakenly provided data for 2N7002 instead of 2N7000. The scheme works perfectly on the Multisim model. However, it is possible that the Multisim circuit modeling program uses simplified models of semiconductor elements (as in many others). At the same time on the graphs of output characteristics https://www.vishay.com/docs/70226/70226.pdf an increase in the drain current relative to zero is observed at a gate voltage above 2.1 V, so the 2N7000 transistor can be used as a threshold element. I have considered Your suggestion regarding the possibility of using a real voltage comparator, such as the LM339, as switching elements. I will send You variants of these schemes to your email address.
Did you notice the graph on page 11-5 of the Vishay datasheet that shows how threshold voltage varies with temperature? E.g., by ~0,1V per 25 degrees?
Thank You, I looked at this graph. There are no ideal semiconductor devices, all their parameters depend on the measurement conditions. On this graph (and usually on other graphs of any technical descriptions), averaged data are given, usually favorable for this semiconductor device. In particular, the graph you are quoting shows data only for the 0.25 mA drain current, although the data obtained for a number of other currents are also of interest. It is likely that the characteristics represented by just one curve on the graph are selected by the manufacturer from many other curves as the most preferred (best) for this device. Yesterday I sent to Your address …@net.chem.unc.edu an email, but it didn’t get through, apparently the address was old. My address: shustov@tpu.ru
“.ru?” You have to be kidding.
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