DC bias voltage VS DC voltage

[Simmon]

What is the difference between a DC bias voltage and a DC voltage?

How do you know when looking at a schematic or when doing measurements on a circuit board that it's a DC bias voltage and not a DC voltage?

What circuits or stages of a circuit have you come across that use DC bias voltages? or needs a DC bias voltage to work?

I can't tell the difference and I don't know when a DC voltage is a bias voltage or it's just a regular DC voltage, how can you tell between the two?

 

[(*steve*)]

That's like asking "What is the difference between a tyre and a spare tyre."

Depending on your perspective they're either exactly the same or very different.

A DC voltage is a DC voltage, and a DC Bias voltage is a DC voltage. A DC bias voltage is a DC voltage used as a bias, but a DC Voltage may not always be used as a bias.

If you're talking about a DC bias voltage as a voltage applied to something to set a certain operating point then the voltage and the characteristics of the device set some operating conditions. The DC Bias voltage is just a DC voltage, but in this particular application is has a certain effect and that is to bias the circuit.

Maybe someone else can state this more clearly.

 

[Simmon]

What circuits or stages of a circuit uses or need a bias voltage?

 

[Simmon]

Does a bias voltage cause the output of that stage to level shift? The bias voltage changes the starting point or zero crossing point which is a level shift Change of the input which will level shift the output voltage?

 

[(*steve*)]

What circuits or stages of a circuit uses or need a bias voltage?

That question is poorly worded. A circuit may need biasing, but it doesn't need a bias voltage.

This is similar to me saying my tomato plants need some 6 foot stakes, and you asking "what other plants need 6 feet?"

The best answer is that you need a bias voltage if the circuit needs biasing.

 

[(*steve*)]

Does a bias voltage cause the output of that stage to level shift?

It may do

The bias voltage changes the starting point or zero crossing point which is a level shift Change of the input which will level shift the output voltage?

This may be true (and is in fact a good description of biasing a class A amplifier). However, biasing may be to a point insufficient to change the output -- you may bias something to a cutoff voltage.

 

[Simmon]

What circuits or stages need biasing? Besides amplifiers circuits

what else needs biasing?

biasing to a cutoff is for transistor switching?

 

[(*steve*)]

What circuits or stages need biasing? Besides amplifiers circuits

what else needs biasing?

biasing to a cutoff is for transistor switching?

I really don't want to do all your homework for you.

Examine Class A, B, and C amplifiers. Then examine transistors used in switching applications.

 

[Simmon]

In electronic books they call a reference voltage a "bias voltage" like on voltage or current sensing comparators

The comparator will "compare" the input signal to the Bias voltage ( reference voltage ) , if it exceeds it or not

How is this a bias voltage?

 

[(*steve*)]

Without seeing the circuit I can't comment on this specific case, but it doesn't sound like a case where I would call it a bias voltage.

Perhaps we can just settle on calling it a voltage

 

[Simmon]

it's for all comparators circuits , they call it a reference voltage that is compared to the input voltage or current

This reference voltage is called a bias voltage

How is it a bias?

 

[sid2286]

Well, I will give it a shot to explain.,

1. DC voltage is a voltage required for the circuit to work.
2. Bias voltage is a voltage required only for a certain section of the circuit, eg. sensors.

so to explain it more clearly, when I apply required voltage to a circuit, the circuit works(say ICs etc) but unless an untill I give a proper bias voltage to the sensor(not all sensors require biasing) the sensor won't work.

Thanks,
S!d

 

[Harald Kapp]

Here's a comprehensive explanation.

 

[Simmon]

Here are the schematics of the resistors i have to select using a decade resistor box, this SAT resistor will bias the op amp comparator

I colored the SAT selected in RED

The SAT resistor sets the DC offset, which is called nulling the output to zero volts

What does the FET do with the SAT resistor?

The Major problem with this circuit is that there is 20 different SAT resistors to select using a resistor decade box.

Each stage of the circuit has feedback paths and input tap off resistors that get summed to other parts of the circuit

When you Null the output of one stage, it changes the voltages and nulls of the stages before, after and in parallel of it.

How can you Null the output of each stage without changing the other stages output nulls and SAT resistor values?

 

[Harald Kapp]

Typically you start from the input and work towards the output. This should minimize any influences of one stage to another - unless you have a feedback loop. In that case you may have to make several passes.

The schematics you show seem to come from some kind of manual. Aren't there trimming instructions in the manual? Follow them and you should be fine.

 

[Simmon]

Yes each stage as feedback loops going to different parts and stages , should I disconnect the feedback loops to do the nulling? Or can leaky coupling caps cause the voltages of stages after to leaky into the stages before? When trying to null out each stage of a circuit that has feedback loops how would u approach this?

 

[Simmon]

What does the FET do please? is it used as a regulator , to keep the reference DC offset voltage from not drifting?

 

[Harald Kapp]

1) If you open the feedback loop, the amplifier may become unstable. Also, if you later reconnect the feedback, the whole circuit will behave differently from the circuit without feedback, So there's in my opinion no use in trying to trim the circuit without feedback.

2) Which FET? There are a few of them in pics #2 and #3. Unfortunately I'm not sure what exactly these FETs do. It looks like they are being used as voltage controlled resistors.

 

[Simmon]

Why did they use a FET as a voltage controlled resistor?

What does using a FET as a voltage controlled resistor do that is different than using a fix resistor value? any advantages?

 

[davenn]

Why did they use a FET as a voltage controlled resistor?

What does using a FET as a voltage controlled resistor do that is different than using a fix resistor value? any advantages?

of course ... you can change its value according to the applied voltage
you cant do that with a normal resistor


D