mikec@wheaties.ai.mit.edu (Mike E. Ciholas) (03/16/90)
I must not understand JFETs yet.
Background...
I am trying to build a circuit that measures current flow very accurately
without inducing a significant voltage drop. The following circuit is shown
on page 2-25 of the Motorola Linear and Interface Integrated Circuits
databook. ("+" indicates connection of crossing wires)
+-----------------------------+
| |
+------|------+ |
| | | |
| | |-----\ |
| | | V+ \ |------|
INPUT-----+--R1--+---|- \ | |
| | \ | LOAD |
R2 | LF355 )--+ | |
| | / | |------|
+----------|+ / | |
| | V- / | |
| |-----/ | GND
2N4092 |--| | |
JFET |<---------|---------+
|--| |
OUTPUT------+ GND
|
R3
|
GND
OUTPUT = R3*(R1/R2)*(current in load)
Typically, R1 is quite small, about .1 ohms. R2 is about 100 ohms and
R3 is about 5K. This gives 5V per amp in load as output.
Now the question...
The way I understand JFETs, N channel types in particular, is that you must
supply a negative gate voltage on the order of -5V to turn them off
(the 2N4092 is a depletion mode JFET).
Since the LF355 is supplied from the INPUT voltage to ground, its output
range must be within this voltage, therefore it cannot turn off the
JFET.
Is this true? Does this circuit work?
I've also seen this circuit in a National databook, and I have seen it with
a bipolar NPN substituted (but isn't the base drive current an error term?).
Mike Ciholas
email: mikec@ai.mit.edu
snail: 289 Highland Ave #108/Somerville, MA 02144
phone: (617) 623 3563
air: N1909C, 1954 Cessna 170Bwhit@milton.acs.washington.edu (John Whitmore) (03/16/90)
In article <7310@rice-chex.ai.mit.edu> mikec@ai.mit.edu (Mike E. Ciholas) writes: > >The way I understand JFETs, N channel types in particular, is that you must >supply a negative gate voltage on the order of -5V to turn them off >(the 2N4092 is a depletion mode JFET). > >Since the LF355 is supplied from the INPUT voltage to ground, its output >range must be within this voltage, therefore it cannot turn off the >JFET. > You are probably correct (the 2N4092 has some dozens of milliamps drain current at zero Vgs); there is a similar circuit in Horowitz and Hill, but using an enhancement-mode MOSFET (any power MOSFET would work as well). Either the negative supply for the op amp must be BELOW ground, or the FET should have zero current at low bias. OR, the circuit will not work at low current input. There remains, however, the possibility that low currents will never need measurement by this circuit... The voltage developed on R3 can reverse bias the JFET into normal operation, but that limits the useful output to about 3 volts minimum (two volts for the FET, one for the op amp output drive circuit). John Whitmore
arnief@tekgvs.LABS.TEK.COM (Arnie Frisch) (03/17/90)
In article <7310@rice-chex.ai.mit.edu>, mikec@wheaties.ai.mit.edu (Mike E. Ciholas) writes: ............ > The way I understand JFETs, N channel types in particular, is that you must > supply a negative gate voltage on the order of -5V to turn them off > (the 2N4092 is a depletion mode JFET). In the circuit you supplied, the voltage across R3 - caused by conduction of the fet, supplies the required negative bias between the gate and source.
richm@amc-gw.amc.com (Rich Moran) (03/17/90)
In article <7310@rice-chex.ai.mit.edu> mikec@ai.mit.edu (Mike E. Ciholas) writes: }I must not understand JFETs yet. } }Background... } }I am trying to build a circuit that measures current flow very accurately }without inducing a significant voltage drop. The following circuit is shown }on page 2-25 of the Motorola Linear and Interface Integrated Circuits }databook. ("+" indicates connection of crossing wires) } } +-----------------------------+ } | | } +------|------+ | } | | | | } | | |-----\ | } | | | V+ \ |------| } INPUT-----+--R1--+---|- \ | | } | | \ | LOAD | } R2 | LF355 )--+ | | } | | / | |------| } +----------|+ / | | } | | V- / | | } | |-----/ | GND } 2N4092 |--| | | } JFET |<---------|---------+ } |--| | }OUTPUT------+ GND } | } R3 } | } GND } }OUTPUT = R3*(R1/R2)*(current in load) } }Typically, R1 is quite small, about .1 ohms. R2 is about 100 ohms and }R3 is about 5K. This gives 5V per amp in load as output. } }Now the question... } }The way I understand JFETs, N channel types in particular, is that you must }supply a negative gate voltage on the order of -5V to turn them off }(the 2N4092 is a depletion mode JFET). } }Since the LF355 is supplied from the INPUT voltage to ground, its output }range must be within this voltage, therefore it cannot turn off the }JFET. } }Is this true? Does this circuit work? } It does work. This circuit is balanced whenever the voltages at the + and - inputs of the LF355 are equal. The voltage at the - input is fixed by the input voltage minus the drop across R1. The voltage at the + input is fixed by a voltage divider consisting of R2, R3 and the JFET. The circuit balances itself by feeding back on the JFET, thereby modifying the voltage divider. With the circuit balanced, the current through R2 will be .001 (.1/100) times the current through R1. Since the current through R2 also passes through R3, the voltage developed across R3 is a direct representation of the current through R2 (and R1). Assume a 1A current through R1. This yields a 1mA current through R2 and R3, and +5V at the output (and the source terminal of the JFET). Note that the JFET will never be completely turned off, it is used as a variable resistance to balance the circuit. Its gate to source junction can be biased as a result of voltage drop across R3. If the JFET IS ever turned completely off, or not biased at all (completely ON), then the circuit is operating outside of its range. There are two limiting factors to the operation of the circuit : First, the input voltage must be at least 5.1 times the LOAD current to allow the circuit to balance. This 5.1/1 ratio can be modified by changing the value of R3, and thereby changing the ratio of volts/amp on the output. If the LOAD current is too high to be measured by the circuit at the given input voltage, the JFET will be biased completely off. For higher currents at lower voltages, R3 should be made smaller. The second limiting factor is low-current operation. If a very small current is drawn by the LOAD, the drop across R3 will be too small to allow the LF355 to bias the JFET. In this case, the JFET will be unbiased, completely on. For low current operation, R3 should be made larger (again, changing the volts/amp ratio on the output). -- ============================================= Rich Moran richm@amc.com =============================================