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In electronics, a 'voltage divider ' is a simple device designed to create a voltage (''V''out) which is proportional to another voltage (''V''in). It is commonly used to create a reference voltage, and may also be used as a signal attenuator at low frequencies. Voltage dividers are also known by the terms 'resistor divider' and 'potential divider'.
A voltage divider referenced to ground is created by connecting two resistors as shown in the following diagram:
The output voltage ''V''out is related to ''V''in as follows:
:
It may be useful to note that ''R''1 and ''R''2 may each comprise many resistors in series.
As a simple example, if ''R''1 = ''R''2 then
:
As a more specific and/or practical example, if ''V''out=6V and ''V''in=9V (both commonly used voltages), then:
:
and by solving using algebra, ''R''2 must be twice the value of ''R''1.
Any ratio between 0 and 1 is possible. That is, using resistors alone it is not possible to either reverse the voltage or increase ''V''out above ''V''in
While voltage dividers may be used to produce very precise reference voltages, they make very poor voltage sources. This is because if a load is connected between the output voltage and ground the effective resistance between ''V''out and ground decreases. A change in the resistance of ''R''2 changes the load voltage, an undesirable situation for a voltage source.
In terms of the above equation, if current flows into a load resistance (through ''V''out), that load resistance must be considered in parallel with ''R''2 to determine the voltage at ''V''out. In this case, the voltage at ''V''out is calculated as follows:
:
where ''R''L is a load resistor in parallel with ''R''2.
Note that for high impedance loads it is possible to use a voltage divider as a voltage source, as long as ''R''1 and ''R''2 have very small values compared to the load. This technique is rarely used, as the power disipated in such a divider would be considerable.
Voltage dividers are often used to produce stable reference voltages. These reference voltages may be used at a device with a high input impedance, such as an op-amp without fear of loading the divider. Alternatively, the reference voltage may be used to set the voltage being produced by a voltage source. A simple way of doing this (for low power applications) is to simply input the reference voltage into the non-inverting input of an op-amp buffer.
A voltage divider is commonly used to set the DC bias of a common emitter amplifier.
A voltage divider is usually thought of as two resistors, but for electronics signals at a given frequency capacitors, inductors, or any combined impedance can be used. For general impedances ''Z''1 and ''Z''2, the voltage becomes
:
For instance, a divider can be made with a resistor and capacitor:
The resistor's impedance is simply its resistance:
:
The capacitor's impedance is a large resistance at low frequencies and a low resistance at high frequencies. The exact formula is:
:
where C is the capacitance of the capacitor, ''j'' is the imaginary unit, and ''ω'' is the frequency of the input voltage which is measured in radians per second. This divider will then have the voltage ratio:
:
The ratio then depends on frequency, in this case decreasing as frequency increases. This circuit is, in fact, a basic (first-order) lowpass filter, or, in the world of audio, a treble-cut filter. The ratio contains an imaginary number, and actually contains both the amplitude and phase shift information of the filter. To extract just the amplitude ratio, calculate the magnitude of the ratio, or just use the reactance of the capacitor instead of the impedance.
★ Potentiometer
★
★ especially Theory of operation
★ Wheatstone bridge
★ Current divider
★ Calculator: voltage divider - loaded and open circuit
★ Lesson In Voltage divider
★ Paul Horowitz and Winfield Hill, ''The Art of Electronics'', Cambridge University Press, 1989.
| Contents |
| Resistor divider rule |
| Voltage divider as a voltage source |
| Use of voltage dividers |
| Impedance divider |
| See also |
| External links |
| References |
Resistor divider rule
A voltage divider referenced to ground is created by connecting two resistors as shown in the following diagram:
Schematic of a resistor divider. ''R''1 is connected to ''V''in and ''V''out, ''R''2 is connected to ''V''out and GND
The output voltage ''V''out is related to ''V''in as follows:
:
It may be useful to note that ''R''1 and ''R''2 may each comprise many resistors in series.
As a simple example, if ''R''1 = ''R''2 then
:
As a more specific and/or practical example, if ''V''out=6V and ''V''in=9V (both commonly used voltages), then:
:
and by solving using algebra, ''R''2 must be twice the value of ''R''1.
Any ratio between 0 and 1 is possible. That is, using resistors alone it is not possible to either reverse the voltage or increase ''V''out above ''V''in
Voltage divider as a voltage source
While voltage dividers may be used to produce very precise reference voltages, they make very poor voltage sources. This is because if a load is connected between the output voltage and ground the effective resistance between ''V''out and ground decreases. A change in the resistance of ''R''2 changes the load voltage, an undesirable situation for a voltage source.
In terms of the above equation, if current flows into a load resistance (through ''V''out), that load resistance must be considered in parallel with ''R''2 to determine the voltage at ''V''out. In this case, the voltage at ''V''out is calculated as follows:
:
where ''R''L is a load resistor in parallel with ''R''2.
Note that for high impedance loads it is possible to use a voltage divider as a voltage source, as long as ''R''1 and ''R''2 have very small values compared to the load. This technique is rarely used, as the power disipated in such a divider would be considerable.
Use of voltage dividers
Voltage dividers are often used to produce stable reference voltages. These reference voltages may be used at a device with a high input impedance, such as an op-amp without fear of loading the divider. Alternatively, the reference voltage may be used to set the voltage being produced by a voltage source. A simple way of doing this (for low power applications) is to simply input the reference voltage into the non-inverting input of an op-amp buffer.
A voltage divider is commonly used to set the DC bias of a common emitter amplifier.
Impedance divider
A voltage divider is usually thought of as two resistors, but for electronics signals at a given frequency capacitors, inductors, or any combined impedance can be used. For general impedances ''Z''1 and ''Z''2, the voltage becomes
:
For instance, a divider can be made with a resistor and capacitor:
Schematic of a voltage divider with a capacitor. Resistor is connected to ''V''in and ''V''out. Cap is connected to ''V''out and GND
The resistor's impedance is simply its resistance:
:
The capacitor's impedance is a large resistance at low frequencies and a low resistance at high frequencies. The exact formula is:
:
where C is the capacitance of the capacitor, ''j'' is the imaginary unit, and ''ω'' is the frequency of the input voltage which is measured in radians per second. This divider will then have the voltage ratio:
:
The ratio then depends on frequency, in this case decreasing as frequency increases. This circuit is, in fact, a basic (first-order) lowpass filter, or, in the world of audio, a treble-cut filter. The ratio contains an imaginary number, and actually contains both the amplitude and phase shift information of the filter. To extract just the amplitude ratio, calculate the magnitude of the ratio, or just use the reactance of the capacitor instead of the impedance.
See also
★ Potentiometer
★
★ especially Theory of operation
★ Wheatstone bridge
★ Current divider
External links
★ Calculator: voltage divider - loaded and open circuit
★ Lesson In Voltage divider
References
★ Paul Horowitz and Winfield Hill, ''The Art of Electronics'', Cambridge University Press, 1989.
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