by Crutschow

• Mosfet Schema
• Mosfet Schematic Explained
• Mosfet Schematic Circuit

When you parallel a battery with another battery or other source, the batteries are often required to be back charged. This can be done with standard diodes, but that gives close to a half volt drop — even with Schottky diodes. This is especially problematic with low-voltage batteries, where that drop is a significant percentage of the battery voltage, noticeably reducing efficiency and battery life.

To minimize this forward drop you can configure a MOSFET as an ideal diode, which has a very low drop in the forward direction (equal to the current times the MOSFET’s ON resistance) while blocking the current in the reverse direction.

Below is the LTspice simulation of a simple ideal-diode MOSFET circuit. It uses inexpensive components consisting of a P-MOSFET (for use in the positive rail) with a dual PNP transistor and two resistors.

How to use an N channel MOSFET (a type of transistor) to turn anything on and off! Also, remember to put a 100k resistor between gate and ground if you want. Some people claim polarity matters they are right sometimes some mosfets are polarity sensitive. For the mosfet unit shown in the 2 pictures here, POLARITY DOES NOT MATTER, check them side by side with the signal wire connected both ways. In both pictures the hotbed was set to preheat PLA, both led blue (D2) and Red (D1) light up and everything functions as it should.

Q1 and Q2 form a current mirror circuit. The indicated values of R1 and R2 cause Q2 to be on and thus M1 off (Vgs ≈0V), when there is no voltage difference between the drain and source of M1. The mirror has a gain of ≈130 from the voltage difference between the two emitters to Q2’s collector voltage change.

In the forward direction (output voltage lower than the battery voltage) the current mirror becomes unbalanced due to the difference in emitter voltages, such as to turn Q2 off, which puts the P-MOSFET gate near ground potential, turning it on. This allows current to flow from the battery to the output (left to right) with a low drop. (MOSFETs conduct equally well in either direction when on.)

When the output voltage becomes slightly higher than the battery voltage, this voltage reversal across the MOSFET unbalances the current mirror in the opposite direction, causing Q2 to turn on. This causes the MOSFET gate voltage to rise, reducing Vgs [V(G,Out) in plot], which turns it off and prevents reverse current flow.

This can be seen in the simulation, as the current only goes out of the V1 battery when the V2 output voltage is lower than the battery voltage, and doesn’t flow in the reverse direction when the output voltage is greater than the battery voltage. The maximum voltage drop, when the battery is providing 2A current is ≈32mV with the MOSFET shown, demonstrating the near ideal diode operation.

The current mirror operation is very sensitive to any offset between the two transistor base-emitter voltages, which could possibly allow some current conduction in the reverse direction. It is thus recommended that a matched transistor pair be used, such as the DMMT3906W shown on the schematic (basically two 2N3906’s in one package), which have their Vbe matched to within 2mV max and are thermally connected.

(The simulation was done with 2N3906‘s which are perfectly matched in the simulation, unlike real life.) The DMMT3906W pair are quite inexpensive, selling for U$0.37 here Reggae drum fills samples. Teri yaadein atif aslam mp3 song 320kbps. , for example. Wow 3 person flying mount.

The P-MOSFET selected should have an on-resistance small enough to give a low voltage drop when conducting the maximum battery load current. If the battery voltage is less than 10V then a logic-level type MOSFET should be used which have gate-source threshold voltages (Vgsth) of less than 2V.

One of these circuits can be used at the output of each battery; however, many are in parallel.

You can read more articles by Electro-Tech-Online “Well-known” member, Crutschow, here.

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• AS and A2:
• MOSFET

_a

Uses

• drive loudspeakers
• amplify radio frequency energy before feeding to the antenna
• drive DC motors. Both speed and direction can be controlled.

_b

Source Followers

• The N Channel FET provides power amplification for the positive part of the AC input.
• The P Channel FET provides power amplification for the negative part of the AC input.
• The voltage gain is 1
• No output coupling capacitor is needed (avoiding the use of a physically big component). Single ended (not push pull) amplifiers need a big output coupling capacitor.
• When there is no input, neither MOSFET is conducting. This saves energy. Single ended amplifiers consume power even when there is no input.
• When there is an AC input, each MOSFET is conducting for only 50% of the time.

_c

Cross Over Distortion

This simple circuit suffers from cross over distortion.

The red trace is the input signal. The blue trace is the output.

• Quite a large input voltage is needed to turn on the FETs, 2 to 4 Volts.
• This has an unwanted side effect. The output is 2 to 4 volts less than the ideal case.
• A positive potential will turn on the top N Channel FET.
• A negative potential will turn on the bottom P Channel FET.
• Small potentials close to zero will turn on neither FET.
• This causes severe cross over distortion, most noticeable with quiet music.
• The amplifier works fairly well for potentials greater than +/- 2 to 4 volts but hardly works at all for lower potentials.

_d

Bias the MOSFETs

This diagram shows simple biasing using diodes and resistors. 0.7 Volts is lost across the diodes so the output will be lower than expected compared with using ideal components. It is possible to use LEDs. In this case about two Volts will be lost.

_e

Adjustable Bias and Quiescent Current

The diagram below is similar but has adjustable biasing. The additional voltage divider resistors, with Rv adjustable are chosen so that both MOSFETS are just on the point of turning on. Rv is adjusted to give a small quiescent current (the current flowing when there is no input signal).

Looking at the graphs, the N Channel MOSFET needs about +3.5V to just start it conducting. The P Channel MOSFET needs -3.5V. The potential difference measured by the voltmeter will be 7 Volts.

Coupling capacitors are needed to get the AC input to the MOSFET gates at the same time as blocking the DC bias voltages. This circuit can not be used to amplify DC signals.

Diodes could be included with the biasing resistors. These would improve the thermal stability of the circuit by tending to shut down an overheating circuit.

The red trace is the input signal. The blue trace is the output. The distortion is reduced.

_f

Use Negative Feedback

• This circuit uses both biasing and negative feedback to improve performance.
• The LEDs have two volts across them. This helps to reduce cross over distortion. This is an unusual way of biasing the MOSFETs but it works.
• The MOSFETS are included in the feedback path.
• The Op Amp voltage follower uses a higher power supply voltage. This allows the MOSFET source follower outputs to swing over a larger range of voltages.

The red trace is the input signal. The blue trace is the output. The distortion has gone.

This push-pull amplifier uses a voltage follower and MOSFET biasing. It runs on + and - 12 Volts and is similar to the diagram above.

• This circuit has a voltage gain of 1 but a much higher power gain (power_out / power_in).
• The Op amp output potential will be just right to ensure that Vout = Vin
• Negative feedback is being used to correct for errors in the output.
• The operational amplifier is wired up as a voltage follower so Vout should track Vin exactly.
• Cross over distortion is minimised.

_g

Push Pull Advantages

• Don't need a large coupling capacitor between the output and the speaker.
• In other types of amplifier, this capacitor limits the low frequency response (high pass filter).

_h

Push Pull Disadvantages

• Cross Over Distortion
• MOSFETs have good high frequency properties. Usually this is an advantage but it makes it easy to build an oscillator capable of high power outputs. The oscillations are likely to be outside the range of human hearing but still able to overheat and destroy speakers, usually the tweeters. Careful design is needed.

Mosfet Schema

_i

Saturation, Clipping, Limiting

• An ideal op amp could provide an infinite output voltage range.
• A very good op amp could provide outputs at least up to the power supply voltages.
• Most op amps fall short by about two volts so with a 12 volt supply, the output would be only ten volts.
• The output should be directly proportional to the input. That is perfectly linear.

The image below shows ideal (black) and non-ideal (red and blue) behaviour including clipping when the op amp is saturated and the output voltage can go no higher.

Amplifiers of any type can not produce output voltages that are larger than the power supply voltages. If the input is too big, the amplifier output will increase until it is nearly equal to the supply voltage. After that the output voltage can not rise any more. The black line shows the amplifier input signal. The red line shows the output from the N Channel MOSFET. The blue line shows the output from the P Channel MOSFET.

_j

RMS Output Power

• The power supply is 20 Volts.
• An 8Ω speaker is being used.
• Decide whether to use 20V (ideal) or 18V (real life) in the calculation. If the exam question does not make it clear which one to use, just say whether you are doing the ideal or real life calculation. Below, the ideal calculation is shown.

Vrms = 0.7 x Vpeak
Power = Vrms² / R
Power = (20 x 0.7)² / 8
Power = 24.5 Watts

This is the theoretical maximum power output.

_k

Real Life Power Output

Mosfet Schematic Explained

In real life, MOSFET push pull source followers are not perfect. The output will be lower than expected because ..

1. The driver op-amp saturates a couple of volts below the power supply voltage.
2. 2 or 3 volts are lost across the gate source junction in the MOSFETs.
3. 0.7 to 4 Volts get lost in the biasing diodes depending on the type of diode used.
4. The MOSFETs have Drain to Source resistance. Energy is lost here.

Mosfet Schematic Circuit

Points 1 to 3 above can be fixed by running the op-amp driver and MOSFET biasing on a higher power supply voltage. As these are low power circuits, this is not too expensive to do.

_l

Falstad Simulations

_m

Simplest Circuit - Bad Crossover Distortion

For the Falstad Circuit Simulation, CTRL+Click Push Pull Source Followers with no Bias and no Negative Feedback
In options, check European Resistors and uncheck Conventional Current.
Alternatively view Push_Pull_No_Bias_No_Feedback.txt.
Save or copy the text on the web page. Import the saved or copied text into the Falstad simulator.
Here is the new HTML5 Simulator Site.

_n

Circuit With Biasing - Improved Crossover Distortion

For the Falstad Circuit Simulation, CTRL+Click Push Pull Source Followers with Bias but no Negative Feedback
In options, check European Resistors and uncheck Conventional Current.
Alternatively view Push_Pull_Bias_No_Feedback.txt.
Save or copy the text on the web page. Import the saved or copied text into the Falstad simulator.
Here is the new HTML5 Simulator Site.

_o

Circuit With Biasing and Negtive Feedback - Minimal Distortion

For the Falstad Circuit Simulation, CTRL+Click Push Pull Source Followers with Bias and Negative Feedback
In options, check European Resistors and uncheck Conventional Current.
Alternatively view Push_Pull_Bias_Feedback.txt.
Save or copy the text on the web page. Import the saved or copied text into the Falstad simulator.
Here is the new HTML5 Simulator Site.

_p

Circuit suffering from Clipping, Saturation or Limiting

This can be eliminated by using a higher power supply voltage as long as all the components can handle this and also the extra waste heat produced.

For the Falstad Circuit Simulation, CTRL+Click Overloaded Push Pull Source Followers
In options, check European Resistors and uncheck Conventional Current.
Click both the switches to double the power supply voltage.
Alternatively view Saturation.txt.
Save or copy the text on the web page. Import the saved or copied text into the Falstad simulator.
Here is the new HTML5 Simulator Site.

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