A RLC oscillator vs class A or B amplifier for EM induction

[Cosma]

Good morning, I have a question that I can't answer. Considering devices for wireless energy transfer, or more generally for electromagnetic induction, why are RLC oscillators used (in particular the capacitor) to charge the coil, and not instead transistor amplifiers, such as in class A or C? Yet the amplifiers could receive the signal from a 555 and then amplify the wave from an external power supply, and then from the collector - emitter, transfer to the coil, why are amplifiers not used and instead RLC oscillators are chosen, that is, why is a capacitor chosen to charge the coil and not the transistor?

 

[berkeman]

Can you post links to some examples of the circuits you've seen for each application? For wireless energy transfer, the efficiency of the transfer will be a main consideration.

 

[DaveE]

The energy oscillating in the inductor is best stored in a capacitor (an LC tank) because that is much more efficient than adding and removing the magnetic field energy via a power supply with active devices. Once the tank is "charged" up, the power supply only has to resupply the losses without processing the total stored energy.

 

[Baluncore]

The inductor is needed for the magnetic coupling. The positive reactance of the inductor, X_L, is neutralised by the negative reactance of a parallel capacitor, Xc, making a resonant LC "tank circuit".

When the reactance; X_L+ Xc = 0 ; the circuit will be resonant, and the LC resonator will circulate energy between the magnetic field of L, and the electric field of C, which will be 90° out of phase, so will not represent real power.

The resistance of that resonant circuit should be minimised to reduce of real power loss; W = I²R .

The resonant circuit will usually be driven by a class-C amplifier, with feedback arranged to oscillate efficiently, at the LC self-resonant frequency.

The inductor in the charger, forms the primary winding of a loosely-coupled transformer, with the secondary winding being the pickup coil of the device being charged. As energy is transferred from the resonant circuit to the secondary load, that energy is replaced by the oscillator/amplifier circuit.

 

[Cosma]

Can you post links to some examples of the circuits you've seen for each application? For wireless energy transfer, the efficiency of the transfer will be a main consideration.

this is the example from which I took inspiration, the doubt came to me when I thought that the speakers are moved by an electrical signal, and the LC mesh acts as a crossover and not as an inductive accumulator, that's why I asked the question

 

[berkeman]

this is the example

Which?

 

[Cosma]

The energy oscillating in the inductor is best stored in a capacitor (an LC tank) because that is much more efficient than adding and removing the magnetic field energy via a power supply with active devices. Once the tank is "charged" up, the power supply only has to resupply the losses without processing the total stored energy.

yes I think this is the right answer, the big difference is the type of resulting wave, the capacitor creates a pulse, which according to Faraday's law is much more efficient than a sinusoidal and linear wave of a transistor, so in the inductive coupling (wpt) to recharge the inductor they use a capacitor, and not a transistor, why is a pulse more efficient than a sinusoid?

 

[Cosma]

Which?

https://www.researchgate.net/public...circuit_for_inductive_wireless_power_transfer

 

[Cosma]

The inductor is needed for the magnetic coupling. The positive reactance of the inductor, X_L, is neutralised by the negative reactance of a parallel capacitor, Xc, making a resonant LC "tank circuit".

When the reactance; X_L+ Xc = 0 ; the circuit will be resonant, and the LC resonator will circulate energy between the magnetic field of L, and the electric field of C, which will be 90° out of phase, so will not represent real power.

The resistance of that resonant circuit should be minimised to reduce of real power loss; W = I²R .

The resonant circuit will usually be driven by a class-C amplifier, with feedback arranged to oscillate efficiently, at the LC self-resonant frequency.

The inductor in the charger, forms the primary winding of a loosely-coupled transformer, with the secondary winding being the pickup coil of the device being charged. As energy is transferred from the resonant circuit to the secondary load, that energy is replaced by the oscillator/amplifier circuit.

so in an inductive coupling you use a capacitor to charge an inductor, because compared to a transistor it produces a pulse (much more efficient for Faraday) compared to a sine wave and linear, right?

 

[DaveE]

so in an inductive coupling you use a capacitor to charge an inductor, because compared to a transistor it produces a pulse (much more efficient for Faraday) compared to a sine wave and linear, right?

Nope. An LC tank circuit has sinusoidal waveforms. This is because that is what these systems require. A single frequency optimized for relatively poor magnetic coupling to the receiving antenna (coil).

I would rethink your statement that capacitors make pulses. There are many different ways capacitors are used in circuits.

 

[Cosma]

Nope. An LC tank circuit has sinusoidal waveforms. This is because that is what these systems desire. A single frequency optimized for relatively poor magnetic coupling to the receiving antenna (coil).

I would rethink your statement that capacitors make pulses. There are many different ways capacitors are used in circuits.

no but, when I was talking about tank circuits, I was talking about amplifiers with active components, speaking instead of your second statement, capacitors produce pulses (switching), which unlike a sinusoid does not have the negative part of the half-wave. anyway, in my opinion this is the reason why capacitors are chosen, but if it is not this, what is it?

 

[Baluncore]

anyway, in my opinion this is the reason why capacitors are chosen, but if it is not this, what is it?

The inductor is needed to generate a magnetic field. The reactance of the inductor must be matched to the reactance of the capacitor, in order for energy to circulate efficiently in the tank circuit.

 

[berkeman]

capacitors produce pulses (switching)

I'm not sure why you keep saying that; it isn't true. Are you familiar with the differential equation that relates voltage and current for a capacitor?
$$i(t) = C \frac{dv(t)}{dt}$$

 

[Cosma]

The inductor is needed to generate a magnetic field. The reactance of the inductor must be matched to the reactance of the capacitor, in order for energy to circulate efficiently in the tank circuit.

yes and I know this because this is how tank circuits work, but why can't I use the transistor? that is, a circuit in which there is the power supply, a signal generator, a bjt transistor (with relative resistances) and the inductor, therefore with the transistor that acts as a signal amplifier for the inductor, why is this configuration not good for wireless charging (electromagnetic induction)?

 

[Cosma]

I'm not sure why you keep saying that; it isn't true. Are you familiar with the differential equation that relates voltage and current for a capacitor?
$$i(t) = C \frac{dv(t)}{dt}$$

Yes i know this equation, buy, im writing you the same message i wrote to Baluncore, "yes and I know this because this is how tank circuits work, but why can't I use the transistor? that is, a circuit in which there is the power supply, a signal generator, a bjt transistor (with relative resistances) and the inductor, therefore with the transistor that acts as a signal amplifier for the inductor, why is this configuration not good for wireless charging (electromagnetic induction)?"

 

[berkeman]

To get the highest efficiency from a driven LC tank circuit, you will want to drive it with a transistor or transistors that alternate between saturation and cutoff. If you operate an amplifier transistor in its active/linear region, you will be dropping a voltage across it as current flows through it, which is power lost. If you operate the transistor in saturation, there is very little voltage drop across it while current flows through it, and in the cutoff region there is very little current flowing through it while there is a voltage across it.

This is why switching power supplies are much more efficient than linear power supplies, and why a Class C amplifier circuit is recommended for driving the LC tank for wireless power transmission. Does that help?

 

[DaveE]

Expanding a bit on @berkeman's response. The nature of the capacitor to respond to the changing inductor current with just the right voltage drop to support a single frequency oscillation without losses is virtually impossible to replicate with devices like transistors that can't store and return energy. Transistors don't store energy, they control the flow of energy either by acting as switches or by dissipating excess energy. Transistors aren't magic, they can't do everything.

For this problem, you have to choose up front if you want a sinusoidal (single frequency) magnetic field or more of a piece-wise linear excitation (like triangle waves and maybe trapezoids). This can be a complicated subject. Honestly most EEs don't have the background for it.

It really is time for you to focus on studying the essentials of analog circuits; the behavior of inductors and capacitors and how they are fundamentally different than switches and resistors. There is no shortcut that I know from studying calculus, simple harmonic oscillators, and the basics of analog circuits, to understand these things. It appears to me that you are stuck trying to understand circuits that you are not yet prepared to deal with.

Asking us repeatedly why you can't do something that doesn't really work is not a great path to learning what does work.

 

[Pisica]

The amplifier is used.
https://www.circuits-diy.com/simple-fm-transmitter-by-using-one-transistor/

The 2n2222 transistor is mounted in a common base amplifier with negative feedback.
C1 puts the base of the transistor to ground for high frequency
C2 brings part of the output signal from the collector of the transistor to the input of the emitter. It is the negative reaction due to the phase shift of the capacitor.

The LC circuit in the collector is the stable emission frequency. It can't be just L without C because it wouldn't have an almost fixed frequency of oscillation.

LC without C would be like a swing trying to swing in the absence of gravity, It doesn't work. Good luck experimenting.

If you used an amplifier with the output in an emission coil, where did you get the audio signal for the amplifier?
the audio player has an RC or quartz or LC oscillator and we still end up with the oscillator

The processor of my laptop that plays music and any audio frequency has a quartz oscillator