# Logic circuits use current source or voltage source?

Main question: Do logic circuits use a current source or a voltage source? My assumption is that they use current sources because the standard is to use voltage as the indicator of input and output values (usually 0V and 5V).

EDIT: There appears to be some confusion about my understanding. So here are my assumptions:

1. I am assuming Ohm's law V = IR, meaning that with a constant current (current source), resistance controls voltage, but with a constant voltage (voltage source), resistance controls current.

2. I am assuming that (approx) 5V output/input means the bit is a 1 and (approx) 0V output/input means the bit is a 0.

3. Circuits change the voltage, by 1) using resistors to control the voltage range of the power, inputs, and output directions, and 2) using voltage controlled transistors to control the actual logic.

Secondary questions:

• If a current source, then would it be safe to assume that using a battery or a 5V/10A power supply (usually voltage sources by my understanding) would be a bad idea with standard logic circuits?

• And what would the amperage of a good current source be for a logic circuit?

• What do you understand under the term "current source"? – Eugene Sh. Jul 26 '18 at 17:44
• Have you ever seen a battery or a wall outlet which is a "current source"? Yet our "logical circuits" are all powered from there. – Eugene Sh. Jul 26 '18 at 17:47
• @Danegraphics OK, this should answer your question fully: allaboutcircuits.com/textbook/digital/chpt-3/… – Eugene Sh. Jul 26 '18 at 17:53
• You're abusing ohm's law. Ohm's law only applies for passive components. Transistors aren't passive, – Tom Carpenter Jul 26 '18 at 18:05
• Actually, most logic circuits consume zero current (ignoring leakage through closed transistors) when all bits are static. Current only flows when bits change. If every single bit required current to flow all the time, modern CPUs would consume megawatts of power. – Dmitry Grigoryev Jul 26 '18 at 19:02

There are two common types:

1) Voltage source switched logic CMOS (Very low static current )

• Although not 0 Ohm voltage sources , logic families exist for 300 Ohm high V (CD4xxx) 50 Ohm 74HCxx and 25 Ohm 74ALCxx

Although dozens of CMOS families now exist, they offer different voltage ranges and speeds. 3~18V lowest speed, 5V standard speed , 3 V lower power or high speed, < 3V etc examples

2) Current Model Logic (CML) aka ECL. at very high speed ( GHz )

• CML is optimized for 50 Ohm sources and 400mV swing.
• usually differential so Vcc current is constant and noise free.

Older TTL is asymmetric current bias but defined by voltage threshold and input current can tolerate floating inputs unlike CMOS but generally low current High"1", higher current low V"0". Input Threshold is 2 Vbe drops.

Logic is always defined by Voltage thresholds with std. margin for noise. so in between "0" & 1" depends on temperature, supply voltage , supplier, tolerances and noise so it is switched rapidly.

Some may argue CMOS logic ( with validity) that CMOS is just Voltage controlled conductors to Vdd and Vss that conduct a known small amount of current in between and nothing unless loaded in either state or a dynamic amount of current according to capacitance load (pF) and frequency of switching. (Ic=CdV/dt)

• So does that mean both are used? If so, which is more common? – Danegraphics Jul 26 '18 at 18:11
• CML is >10x speed and 100x the cost, thus fewer types – Sunnyskyguy EE75 Jul 26 '18 at 18:17
• So most manufacturers of logic circuits use constant voltage sources instead of constant current sources? – Danegraphics Jul 26 '18 at 18:19
• They switch to each Vdd,Vss or V+,0V. that's all. – Sunnyskyguy EE75 Jul 26 '18 at 18:25

Look at a typical 7400 NAND gate (which is similar to many digital output stages) , with the output stage being formed by Q3 and Q4:

It's not strictly a voltage source, ideal voltage sources would keep the voltage constant as we increase the load (the current) we see the voltage drop. If it were a voltage source, the voltage would not drop as we increased the load (as shown below in the graph. If it were a voltage source, you'd see a straight horizontal line no matter the load

It's not strictly a current source, ideal current sources would keep the current constant no matter the load and the voltage would vary on the output to keep the current constant, we don't see that either.

So what is going on? The answer is transistors can be thought of as resistors or varying loads. Logic circuit output stages are more like resistors, or switches with resistance.

Although we can configure transistors to function like current sources, this requires feedback, there is no feedback in most digital output stages.

All logic integrated circuits use constant voltage supply (Vdd), regardless if the internal logic is current-switch-driven, or use voltage-controlled switches (or "variable resistors", or "voltage-controlled transistors" if you wish).

The difference is that CML/ECL logic consumes about the same (usually huge) current from Vdd regardless if it is doing some logic operations or not, while consumption of CMOS-based logic ICs depends on how frequently the internal gates switch. In static mode CMOS-based ICs consume nearly nothing, and consumption grows linearly with increased clock rate. This allows for more efficient use of energy by trading consumption for on-demand performance, which allows for "low-power modes" and eventually design of very energy-efficient computation devices.

In no case industrial digital logic ICs use power source of constant-current type, partly because most internal and external interfaces are defined as certain voltage levels, which would be difficult to maintain if not using constant-voltage Vdd.