Current source

The simplest non-ideal current source consists of a voltage source in series with a resistor. The amount of current available from such a source is given by the ratio of the voltage across the voltage source to the resistance of the resistor (Ohm's law; I = V/R). This value of current will only be delivered to a load with zero voltage drop across its terminals (a short circuit, an uncharged capacitor, a charged inductor, a virtual ground circuit, etc.) The current delivered to a load with nonzero voltage (drop) across its terminals (a linear or nonlinear resistor with a finite resistance, a charged capacitor, an uncharged inductor, a voltage source, etc.) will always be different. It is given by the ratio of the voltage drop across the resistor (the difference between the exciting voltage and the voltage across the load) to its resistance.

For a nearly ideal current source, the value of the resistor should be very large but this implies that, for a specified current, the voltage source must be very large (in the limit as the resistance and the voltage go to infinity, the current source will become ideal and the current will not depend at all on the voltage across the load). Thus, efficiency is low (due to power loss in the resistor) and it is usually impractical to construct a 'good' current source this way. Nonetheless, it is often the case that such a circuit will provide adequate performance when the specified current and load resistance are small. For example, a 5 V voltage source in series with a 4.7 kΩ resistor will provide an approximately constant current of 1 mA ± 5% to a load resistance in the range of 50 to 450 Ω.

A Van de Graaff generator is an example of such a high voltage current source. It behaves as an almost constant current source because of its very high output voltage coupled with its very high output resistance and so it supplies the same few microamperes at any output voltage up to hundreds of thousands of volts (or even tens of megavolts) for large laboratory versions.

In these circuits the output current is not monitored and controlled by means of negative feedback.

They are implemented by active electronic components (transistors) having current-stable nonlinear output characteristic when driven by steady input quantity (current or voltage). These circuits behave as dynamic resistors changing their present resistance to compensate current variations. For example, if the load increases its resistance, the transistor decreases its present output resistance (and vice versa) to keep up a constant total resistance in the circuit.

Active current sources have many important applications in electronic circuits. They are often used in place of ohmic resistors in analog integrated circuits (e.g., a differential amplifier) to generate a current that depends slightly on the voltage across the load.

The common emitter configuration driven by a constant input current or voltage and common source (common cathode) driven by a constant voltage naturally behave as current sources (or sinks) because the output impedance of these devices is naturally high. The output part of the simple current mirror is an example of such a current source widely used in integrated circuits. The common base, common gate and common grid configurations can serve as constant current sources as well.

A JFET can be made to act as a current source by tying its gate to its source. The current then flowing is the I_DSS of the FET. These can be purchased with this connection already made and in this case the devices are called current regulator diodes or constant current diodes or current limiting diodes (CLD). Alternatively, an enhancement-mode N-channel MOSFET (metal–oxide–semiconductor field-effect transistor) could be used instead of a JFET in the circuits listed below for similar functionality.

An example: bootstrapped current source.[1]

The simple resistor passive current source is ideal only when the voltage across it is zero; so voltage compensation by applying parallel negative feedback might be considered to improve the source. Operational amplifiers with feedback effectively work to minimise the voltage across their inputs. This results in making the inverting input a virtual ground, with the current running through the feedback, or load, and the passive current source. The input voltage source, the resistor, and the op-amp constitutes an "ideal" current source with value, I_OUT = V_IN/R. The transimpedance amplifier and an op-amp inverting amplifier are typical implementations of this idea.

The floating load is a serious disadvantage of this circuit solution.

A typical example are Howland current source[2] and its derivative Deboo integrator.[3] In the last example (Fig. 1), the Howland current source consists of an input voltage source, V_IN, a positive resistor, R, a load (the capacitor, C, acting as impedance Z) and a negative impedance converter INIC (R₁ = R₂ = R₃ = R and the op-amp). The input voltage source and the resistor R constitute an imperfect current source passing current, I_R through the load (Fig. 3 in the source). The INIC acts as a second current source passing "helping" current, I_−R, through the load. As a result, the total current flowing through the load is constant and the circuit impedance seen by the input source is increased. However the Howland current source isn't widely used because it requires the four resistors to be perfectly matched, and its impedance drops at high frequencies.[4]

The grounded load is an advantage of this circuit solution.

They are implemented as a voltage follower with series negative feedback driven by a constant input voltage source (i.e., a negative feedback voltage stabilizer). The voltage follower is loaded by a constant (current sensing) resistor acting as a simple current-to-voltage converter connected in the feedback loop. The external load of this current source is connected somewhere in the path of the current supplying the current sensing resistor but out of the feedback loop.

The voltage follower adjusts its output current I_OUT flowing through the load so that to make the voltage drop V_R = I_OUTR across the current sensing resistor R equal to the constant input voltage V_IN. Thus the voltage stabilizer keeps up a constant voltage drop across a constant resistor; so, a constant current I_OUT = V_R/R = V_IN/R flows through the resistor and respectively through the load.

If the input voltage varies, this arrangement will act as a voltage-to-current converter (voltage-controlled current source, VCCS); it can be thought as a reversed (by means of negative feedback) current-to-voltage converter. The resistance R determines the transfer ratio (transconductance).

Current sources implemented as circuits with series negative feedback have the disadvantage that the voltage drop across the current sensing resistor decreases the maximal voltage across the load (the compliance voltage).

The internal structure of a current limiting diode

The simplest constant-current source or sink is formed from one component: a JFET with its gate attached to its source. Once the drain-source voltage reaches a certain minimum value, the JFET enters saturation where current is approximately constant. This configuration is known as a constant-current diode, as it behaves much like a dual to the constant voltage diode (Zener diode) used in simple voltage sources.

Due to the large variability in saturation current of JFETs, it is common to also include a source resistor (shown in the adjacent image) which allows the current to be tuned down to a desired value.

Figure 4: Typical BJT constant current source with negative feedback

In this bipolar junction transistor (BJT) implementation (Figure 4) of the general idea above, a Zener voltage stabilizer (R1 and DZ1) drives an emitter follower (Q1) loaded by a constant emitter resistor (R2) sensing the load current. The external (floating) load of this current source is connected to the collector so that almost the same current flows through it and the emitter resistor (they can be thought of as connected in series). The transistor, Q1, adjusts the output (collector) current so as to keep the voltage drop across the constant emitter resistor, R2, almost equal to the relatively constant voltage drop across the Zener diode, DZ1. As a result, the output current is almost constant even if the load resistance and/or voltage vary. The operation of the circuit is considered in details below.

A Zener diode, when reverse biased (as shown in the circuit) has a constant voltage drop across it irrespective of the current flowing through it. Thus, as long as the Zener current (I_Z) is above a certain level (called holding current), the voltage across the Zener diode (V_Z) will be constant. Resistor, R1, supplies the Zener current and the base current (I_B) of NPN transistor (Q1). The constant Zener voltage is applied across the base of Q1 and emitter resistor, R2.

Voltage across R₂ (V_R2) is given by V_Z − V_BE, where V_BE is the base-emitter drop of Q1. The emitter current of Q1 which is also the current through R2 is given by

${\displaystyle I_{\text{R2}}(=I_{\text{E}}=I_{\text{C}})={\frac {V_{\text{R2}}}{R_{\text{2}}}}={\frac {V_{\text{Z}}-V_{\text{BE}}}{R_{\text{2}}}}.}$

Since V_Z is constant and V_BE is also (approximately) constant for a given temperature, it follows that V_R2 is constant and hence I_E is also constant. Due to transistor action, emitter current, I_E, is very nearly equal to the collector current, I_C, of the transistor (which in turn, is the current through the load). Thus, the load current is constant (neglecting the output resistance of the transistor due to the Early effect) and the circuit operates as a constant current source. As long as the temperature remains constant (or doesn't vary much), the load current will be independent of the supply voltage, R1 and the transistor's gain. R2 allows the load current to be set at any desirable value and is calculated by

${\displaystyle R_{\text{2}}={\frac {V_{\text{Z}}-V_{\text{BE}}}{I_{\text{R2}}}}}$

where V_BE is typically 0.65 V for a silicon device.[5]

(I_R2 is also the emitter current and is assumed to be the same as the collector or required load current, provided h_FE is sufficiently large). Resistance R₁ is calculated as

${\displaystyle R_{\text{1}}={\frac {V_{\text{S}}-V_{\text{Z}}}{I_{\text{Z}}+K\cdot I_{\text{B}}}}}$

where K = 1.2 to 2 (so that R_R1 is low enough to ensure adequate I_B),

${\displaystyle I_{\text{B}}={\frac {I_{\text{C}}}{h_{FE,{\text{min}}}}}}$

and h_FE,min is the lowest acceptable current gain for the particular transistor type being used.

Figure 5: Typical constant current source (CCS) using LED instead of Zener diode

The Zener diode can be replaced by any other diode; e.g., a light-emitting diode LED1 as shown in Figure 5. The LED voltage drop (V_D) is now used to derive the constant voltage and also has the additional advantage of tracking (compensating) V_BE changes due to temperature. R₂ is calculated as

${\displaystyle R_{\text{2}}={\frac {V_{\text{D}}-V_{\text{BE}}}{I_{\text{R2}}}}}$

and R₁ as

${\displaystyle R_{\text{1}}={\frac {V_{\text{S}}-V_{\text{D}}}{I_{\text{D}}+K\cdot I_{\text{B}}}}}$, where I_D is the LED current

Figure 6: Typical constant current source (CCS) with diode compensation

Temperature changes will change the output current delivered by the circuit of Figure 4 because V_BE is sensitive to temperature. Temperature dependence can be compensated using the circuit of Figure 6 that includes a standard diode, D, (of the same semiconductor material as the transistor) in series with the Zener diode as shown in the image on the left. The diode drop (V_D) tracks the V_BE changes due to temperature and thus significantly counteracts temperature dependence of the CCS.

Resistance R₂ is now calculated as

${\displaystyle R_{2}={\frac {V_{\text{Z}}+V_{\text{D}}-V_{BE}}{I_{\text{R2}}}}}$

Since V_D = V_BE = 0.65 V,[6]

${\displaystyle R_{2}={\frac {V_{\text{Z}}}{I_{\text{R2}}}}}$

(In practice, V_D is never exactly equal to V_BE and hence it only suppresses the change in V_BE rather than nulling it out.)

R₁ is calculated as

${\displaystyle R_{1}={\frac {V_{\text{S}}-V_{\text{Z}}-V_{\text{D}}}{I_{\text{Z}}+K\cdot I_{\text{B}}}}}$

(the compensating diode's forward voltage drop, V_D, appears in the equation and is typically 0.65 V for silicon devices.[6])

Note that this only works well if DZ1 is a reference diode or another stable voltage source. Together with 'normal' Zener diodes especially with lower Zener voltages (<5V) the diode might even worsen overall temperature dependency.

Series negative feedback is also used in the two-transistor current mirror with emitter degeneration. Negative feedback is a basic feature in some current mirrors using multiple transistors, such as the Widlar current source and the Wilson current source.

One limitation with the circuits in Figures 5 and 6 is that the thermal compensation is imperfect. In bipolar transistors, as the junction temperature increases the V_be drop (voltage drop from base to emitter) decreases. In the two previous circuits, a decrease in V_be will cause an increase in voltage across the emitter resistor, which in turn will cause an increase in collector current drawn through the load. The end result is that the amount of 'constant' current supplied is at least somewhat dependent on temperature. This effect is mitigated to a large extent, but not completely, by corresponding voltage drops for the diode, D1, in Figure 6, and the LED, LED1, in Figure 5. If the power dissipation in the active device of the CCS is not small and/or insufficient emitter degeneration is used, this can become a non-trivial issue.

Imagine in Figure 5, at power up, that the LED has 1 V across it driving the base of the transistor. At room temperature there is about 0.6 V drop across the V_be junction and hence 0.4 V across the emitter resistor, giving an approximate collector (load) current of 0.4/R_e amps. Now imagine that the power dissipation in the transistor causes it to heat up. This causes the V_be drop (which was 0.6 V at room temperature) to drop to, say, 0.2 V. Now the voltage across the emitter resistor is 0.8 V, twice what it was before the warmup. This means that the collector (load) current is now twice the design value! This is an extreme example of course, but serves to illustrate the issue.

Current limiter with NPN transistors

The circuit to the left overcomes the thermal problem (see also, current limiting). To see how the circuit works, assume the voltage has just been applied at V+. Current runs through R1 to the base of Q1, turning it on and causing current to begin to flow through the load into the collector of Q1. This same load current then flows out of Q1's emitter and consequently through R_sense to ground. When this current through R_sense to ground is sufficient to cause a voltage drop that is equal to the V_be drop of Q2, Q2 begins to turn on. As Q2 turns on it pulls more current through its collector resistor, R1, which diverts some of the injected current in the base of Q1, causing Q1 to conduct less current through the load. This creates a negative feedback loop within the circuit, which keeps the voltage at Q1's emitter almost exactly equal to the V_be drop of Q2. Since Q2 is dissipating very little power compared to Q1 (since all the load current goes through Q1, not Q2), Q2 will not heat up any significant amount and the reference (current setting) voltage across R_sense will remain steady at ≈0.6 V, or one diode drop above ground, regardless of the thermal changes in the V_be drop of Q1. The circuit is still sensitive to changes in the ambient temperature in which the device operates as the BE voltage drop in Q2 varies slightly with temperature.

Figure 7: Typical op-amp current source.

The simple transistor current source from Figure 4 can be improved by inserting the base-emitter junction of the transistor in the feedback loop of an op-amp (Figure 7). Now the op-amp increases its output voltage to compensate for the V_BE drop. The circuit is actually a buffered non-inverting amplifier driven by a constant input voltage. It keeps up this constant voltage across the constant sense resistor. As a result, the current flowing through the load is constant as well; it is exactly the Zener voltage divided by the sense resistor. The load can be connected either in the emitter (Figure 7) or in the collector (Figure 4) but in both the cases it is floating as in all the circuits above. The transistor is not needed if the required current doesn't exceed the sourcing ability of the op-amp. The article on current mirror discusses another example of these so-called gain-boosted current mirrors.

Figure 8: Constant current source using the LM317 voltage regulator

The general negative feedback arrangement can be implemented by an IC voltage regulator (LM317 voltage regulator on Figure 8). As with the bare emitter follower and the precise op-amp follower above, it keeps up a constant voltage drop (1.25 V) across a constant resistor (1.25 Ω); so, a constant current (1 A) flows through the resistor and the load. The LED is on when the voltage across the load exceeds 1.8 V (the indicator circuit introduces some error). The grounded load is an important advantage of this solution.

Nitrogen-filled glass tubes with two electrodes and a calibrated Becquerel (fissions per second) amount of ²²⁶Ra offer a constant number of charge carriers per second for conduction, which determines the maximum current the tube can pass over a voltage range from 25 to 500 V.[7]