Understanding the difference between voltage, current and power amplifiers

Thread Starter

Spacerat

Joined Aug 3, 2015
25
Hello Forum,
I am trying to grasp the difference between voltage, current, power amplifiers... I understand the gain in all three scenarios (the ratio between output and input quantities, V_out/V_in, I_out/I_in and P_out/ P_in).

For example, it is possible to build a circuit using BJT transistors (which I consider current amplifiers) that works like a current amplifier but also like a voltage amplifier. What is the difference? Does it depend on the input and output impedances of the amplifier circuit if we call it voltage or current amplifier? For example, if the input impedance is large, will the input be a voltage? And if an amplifier has low output impedance, will generate a larger output current... I am confused on these concepts....

Power is P=IV so to amplify power we can either amplify both I and V or either one while keeping the other variable constant.

There are also devices that have an input current and amplify the output voltage...
 
it's a question of what's being controlled. Look up "electronic load".

An ideal voltage source has 0 output Z.
An ideal current source has infinate output Z

The controlled parameter for a heater should be temperature. The control element should control power.

An audio amplifier is designed within various limits. A max voltage, a max current which implies a max load and max power.

Power amplifiers do exist and they might be called "linears" and used with RF frequencies.
 

Papabravo

Joined Feb 24, 2006
13,456
Like many folk new to electronics you might be tempted to think, that in some cases, an amplifier creates something out of nothing. I assure this is very far from the truth. At this stage of your experience I would suggest that you substitute the word "copier" for the word "amplifier".

Let me take this a bit further:

A voltage "copier" takes an input signal of some peak AC voltage, say 100 mVAC, and in conjunction with a power supply, makes a copy of that input signal with a peak AC voltage of 10 VAC. The current can be bigger or smaller or unchanged.

A current "copier" takes an input current of some peak AC current, 1 mA AC, and in conjunction with a power supply, makes a copy of that input current wit a peak AC current of 500 mA AC. The output voltage can be bigger or smaller or unchanged.

A "power" copier takes a signal with a given "input power" in watts, saw 100 mWatts, and in conjunction with a power supply, makes a copy of that input signal with a power of 100 Watts, The Output Voltage and Output Current can be anything so long as the product is 100 Watts.

When you look at various single device amplifier types called Common Emitter, Common Collector, and Common Base, you should pay attention to which parameters have gain and which ones have attenuation, and which ones remain substantially the same.
 

MisterBill2

Joined Jan 23, 2018
5,738
Hello Forum,
I am trying to grasp the difference between voltage, current, power amplifiers... I understand the gain in all three scenarios (the ratio between output and input quantities, V_out/V_in, I_out/I_in and P_out/ P_in).

For example, it is possible to build a circuit using BJT transistors (which I consider current amplifiers) that works like a current amplifier but also like a voltage amplifier. What is the difference? Does it depend on the input and output impedances of the amplifier circuit if we call it voltage or current amplifier? For example, if the input impedance is large, will the input be a voltage? And if an amplifier has low output impedance, will generate a larger output current... I am confused on these concepts....

Power is P=IV so to amplify power we can either amplify both I and V or either one while keeping the other variable constant.

There are also devices that have an input current and amplify the output voltage...
Transistors are a device, and mostly they control current. It is the complete circuit that does the amplifying, and so it is the whole circuit that provides power, not just a component. And the very first rule is that you never get more power out of a circuit than is put in to the circuit. AND there are NO devices that amplify current. There are circuits that provide amplification of an input current by controlling another power source. But circuits do not create power.
 

LvW

Joined Jun 13, 2013
957
Hello Forum,
I am trying to grasp the difference between voltage, current, power amplifiers... I understand the gain in all three scenarios (the ratio between output and input quantities, V_out/V_in, I_out/I_in and P_out/ P_in).

For example, it is possible to build a circuit using BJT transistors (which I consider current amplifiers) that works like a current amplifier but also like a voltage amplifier.
.............................
In general, the term "amplifier" is somewhat misleading.
In reality, you always need a DC voltage source and the "amplifier" does nothing else than to convert this DC voltage (or DC power) into an output quantity (current or voltage or power) controlled by a corresponding input quantity.

The conversion can be:
* V-V (opamp)
* I-V (transimpedance amplifier)
* V-I (OTA, BJT, FET)
* I-I (current conveyor)

(As far as power is concerned, it is common practice not to use the term "power amplifier" but instead "power stage")

Comment: Contrary to some false explanations of the principle of operation, the bipolar transistor is an element in which the output current (Ie resp. Ic) is controlled and determined by the input voltage (Vbe). There is much evidence for this.
 

Thread Starter

Spacerat

Joined Aug 3, 2015
25
Hello and thank you for your helpful replies. I like the analogy of an amplifier as a "copier": using externally supplied energy, the amplifies shapes that supplied energy in the form of the input (enlarged replica).

  • I guess I my confusion may be rooted in the idea that a signal (something that carries/maps information) can be a current signal and voltage signal. While reading about sensors (temperature, pressure, light, motion, etc.), I learned that we can talk about "current signals" and "voltage signals" and run into the two ranges 4-20mA and 0-10V in relation to sensors...
  • I guess most (all?) sensors produce a current response, i.e. a current signal, which is correlated with the measured physical quantity? Current signals are less sensitive to noise over long distances...What are examples of sensors that output a voltage signal instead of a current signal? Also, I don't think the sensor I played with with Arduino necessarily and automatically produce currents from 4-20mA...

  • There are circuits called voltage-to-current converters and current-voltage converters. That sounds confusing since current and voltage are different things. I guess these circuits convert a current type signal into a voltage type signal and vice versa. After all, when a current signal passes through a resistor, we could trivially measure the potential difference across the resistor and get the corresponding voltage signal...I guess I am still a little confused on what is means by a current vs voltage signal. Maybe once I understand that, a device that converts I to V, like LvW suggests, may not be as strange to me anymore...

  1. In terms of amplification, I guess what is important is that power, the product IV, is somehow amplified at the amplifier output. there are several scenarios:

1) V increases, I stays constant, P=IV increases
2) V constant, I increases, P=IV increases
3) Both V and I increase, P=IV increases
4) V increases, I decreases, P=IV increases
5) V decreases, I increases, P=IV increases

A transformer changes I and V at the same time but the gain is =<1. That is not an amplifier....



 

MisterBill2

Joined Jan 23, 2018
5,738
"I learned that we can talk about "current signals" and "voltage signals" and run into the two ranges 4-20mA and 0-10V in relation to sensors."
What you are referencing is two major standards for the analog interface of industrial transducers. Those numbers bear no relationship to the operation of the actual sensing element. Those two standards allow equipment to work with equipment from different producers with very few problems, and those two ranges allow controls and instrumentation folks to work around a lot of electrical noise problems. So now it is clear where the confusion comes from. You got into the textbook at chapter 17, and missed all of the 439 pages of the basics that things are built on. Oh Wow!!
 
I guess I am still a little confused on what is means by a current vs voltage signal. Maybe once I understand that, a device that converts I to V, like LvW suggests, may not be as strange to me anymore...
I don't know if this will help or not. Sensor signals need to be "conditioned".

While doing that, I'll throw a large curve ball. https://en.wikipedia.org/wiki/Fieldbus

I'll also throw this out: https://en.wikipedia.org/wiki/Modbus

No voltage/current there. it's digital.

Now let's revisit current and voltage.

The 0-20 mA and 4-20 mA come from 0-5 and 1-5V signals usually. Current can be transmitted over large distances. The 4mA is used to power the sensor. The voltages might be a little higher. There a passive sensors and active sensors and ones that use voltages larger than 5V.

Voltage sensors are a PITA. They are susceptable to noise and ground loops. Twisted pair shielded cables are the norm. Why? The shield protects against RFI. The twists EMI providing the receiving end is differential. A magnetic field disturbs a wire in a positive way and the other in a way that cancels.

You can go connecting the commons of all of the voltage sensors when they are hundreds of feet apart.

So, here is a very common scenereo of old school. 0-5V sets the temperature between 0 and 50C. You can read the temperature as a 0-5V signal.

When I use both, I create ground loops or commons at different potentials.

0-5V is bad because of what's called A/D converter quantization error. Bumping the output to 1V helps that a lot.

Let; convert the 1-5V output to a current with a 250 ohm (249 really) to 4-20 mA. When it reaches the computer, I put another 249 ohm resistor there and convert it o a local voltage of 1-5V. i measure that voltage differentially or pseudo-differentially and we are all happy. No ground loops.

pseudo-differential means the source is grounded so it's an A-GND+GND+B or A-B.

Now if I want to change the setpoint of the controller, lets use a 4-20 mA signal. the 4 mA can tell me if there is a break in the wiring and the resistor at th eother end creates a 1-5 signal from the 4-20 mA signal.

Be careful because there is a 20 mA current loop serial communication thingy out there.

Everybody is happy.

Now, say someone has a bunch of variable power supplies they (me) like to control. The way the power supply is designed is that COMMON is the positive output and the negative side has to be connected to earth.
 
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MisterBill2

Joined Jan 23, 2018
5,738
That whole discussion in post #8 is about instrumentation systems and not about basic electrical theory. So while it is correct, it is not the part that you need to learn about first, unless you are applying for a job as an instrumentation tech.
First you need to learn about ohm's law, which is the relationship between voltage, current, and resistance. That is the foundation. And it is not that complex. Then comes circuit theory, which is that current flows in a path and back to the source. That is the summary, the details are more complicated. All of the discussion about instrumentation and the sensors used there is a different world from the basics.
 

sparky 1

Joined Nov 3, 2018
116
The properties of the transistor in an amplifier arrangement utilize both the flow of charge and the potential differences in the media's electric field. We can use the terminology so that an electric current is the flow of charge and the voltage is the potential difference in charge. A transistor circuit as an AC amplifier can be designed with values needed to match the impedance of the AC amplifier circuit's output to the load.
 
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LvW

Joined Jun 13, 2013
957
"There are circuits called voltage-to-current converters and current-voltage converters. That sounds confusing since current and voltage are different things."

Yes - and that is the reason we are using the term "converter". Two quantities of the same kind are not "converted" into each other....in this case, we speak about "amplification".

I guess these circuits convert a current type signal into a voltage type signal and vice versa.
...............
I guess I am still a little confused on what is means by a current vs voltage signal. Maybe once I understand that, a device that converts I to V, like LvW suggests, may not be as strange to me anymore..."


Well, I can imagine what you mean...
What is a "current-to-voltage" converter?
It is an active circuit with a very low input resistance and a very low output resistance.
Let as assume that a voltage source with a source resistance of 1kohm is connected to such a device that has an input resistance of 1 ohm only.

Because current is always the RESULT of a voltage, the tiny voltage drop across this 1Ohm-resistor is further processed (amplifid) within the circuit.
However, on the other hand, the current into the device is practically determined by the voltage source (and the 1k resistor) and does practicaly not depend on the input resistance. Therefore, it is common practice to say that the input signal is a current - although internally the voltage drop across the 1ohm resistor may be used as a signal quantity.
Something similar applies to the output.

When the ouput voltage is nearly independent on the connected load (due to a very low ouput resistance) we speak about a voltage output. When the opposite is true (large output resistance) and the ouput voltage is primarily detrmined by the load (because the ouput current is primarily determined by the output resistance) we speak about a current output.

As another example, let`s assume that the output resustance is 1kOhm. For a load of 1...10 ohms the device wold provide (primarily) a current output and for load resistors of >50...100 kOhms we would speak of a voltage output.

Fazit: The signal quantities (voltage or current) are something like "labor jargon" and indicate which of both quantities are to be considered as "constant". But this is - more or less - valid within a certain operating range only....

Additional comment: Some people even say that an ohmic resistor Rx would be simple "current-to-voltage" converter (a current through the resistor produces a corresponding voltage). But - physically spoken - this is wrong. A current can never produce a voltage...in contrary - a current is always the result of an applied voltage. In fact, we have nothing else than a voltage divider between the large input resistance Ri of the driving source and the resistor Rx.
 
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sparky 1

Joined Nov 3, 2018
116
An AC transistor amplifier ( in the case of power) can have an additional stage buffer or level shift either current or voltage or both. The practical (being real also and not ideal) AC transistor signal amplifier is just a sub circuit having two ports;
These ports normally labeled input and output. The signal amplifier is extended with a buffer to better suite the nature of the load.

In some cases the 2 port amplifier uses a signal transistor. If the power requirement is greater then a buffer can be added using an appropriate power transistor. The buffer circuit might have a separate source and the signal source may need isolation.

I saw more confusion in past years as projects using a single stage have many variants. Certain trendy circuits for a while affected the internet search engine that sell products rather than educate. Run your house on a single stage joule thief was considered politically correct and those trolling the electronic forums had concealed identities contributed to downgrading electronic education.

Another diversion from the original intent for a simple power circuit became a game of who can use the least amount of parts. Thank you neophites, the result for many beginners was a lack of precision or narrowed operating condition.

The internet search engine is now being used to sell rather than educate. The youtube videos are not rated for educational quality but are rated on likes. The news media is also controlled by the nephalim giants polarizing by selection. There are foreign countries and universities whose motive is to take down the technical progress of the free world so the confusion sometimes misinformation. Why we keep asking what the application is for allows getting down to specifics.
 
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