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34

The term you are looking for is digital potentiometer. Search for parts to find one that fits your needs. You can get them with up to 10 Bits of resolution. Be aware that on power-up the value might be at min or max until you send the needed value via the digital interface.


30

There are downfalls with choosing very large resistors and very small resistors. These usually deal with the non-ideal behavior of components (namely Op-Amps), or other design requirements such as power and heat. Small resistors means that you need a much higher current to provide the appropriate voltage drops for the Op-amp to work. Most op amps are able ...


25

Gain/Bandwidth product, you want maybe 50KHz bandwidth at 60dB (1,000 times), so you need somewhere around 50MHz, gain/bandwidth product (And more would lower HF distortion)... Make it 80dB and now you need 500MHz GBP, which is getting difficult if you want low noise down near DC (And is getting really bad news to stabilise at low gain). Also consider that ...


19

An op-amp is not required. Given the minimal requirements given, a voltage divider will do. Gain = 0.5. simulate this circuit – Schematic created using CircuitLab If you need to buffer the output, you can just place a unity gain op-amp at the output of the voltage divider.


17

Don't bother. 2.5 W is unlikely to hurt a 2 W speaker. It will just cause distortion. Somewhere in your audio chain there must surely be a volume control. Turn down the volume to the point where the sound from the speaker isn't distorted. The 2.5 W rating of a power amp is what it can put out, given the right signal and load. Put in lower volume, and ...


17

It seems you're trying to build a programmable gain amplifier. Programmable resistors, as jusaca says, exist in the shape of digital potentiometers. However, there's also programmable gain amplifiers that you can buy like that; look for PGAs with your favourite electronics distributor (farnell, digikey, mouser, …). Both devices don't have infinite bandwidth (...


15

It's simple: manufacturers make what customers will buy! It's the same reason why Ferrari won't put a trailer attachment at the rear of their cars... Price is a very important part of this, of course, and price is tied to silicon area, process, yield, and of course packaging. For example, an opamp with +/-12V supplies and 1A output current will dissipate ...


14

One reason is that the transistor gain is degraded at high frequencies. To pick a specific example, the ON semiconductor BC546 has a gain-bandwidth product (GBP) of 100MHz at 1mA collector current (see figure 6 in the linked datasheet). This means that at a frequency of 27MHz, the current gain (beta) is about 100MHz/27MHz = 3.7, not 100. At 27MHz, stray ...


14

There is the matter of GBW(gain-bandwidth product), so a single stage is improbable with good performance. It's not enough to just squeak through in bandwidth you also want enough gain to reduce the distortion and get accurate reproduction with flat response (though arguably distortion at more than about 10kHz is unimportant for human hearing). Of course you ...


13

It's just a convention. Decibels always refer to power, and power is proportional to voltage squared and current squared. The math works like this: $$\begin{align} A_{v,dB} &= 10\cdot\log \frac{V_o^2}{V_i^2} \\ &= 10\cdot\log \left(\frac{V_o}{V_i}\right)^2 \\ &= 10\cdot 2 \cdot\log \frac{V_o}{V_i} \end{align} $$ EDIT: Power is ...


13

Your schematic should be as following: simulate this circuit – Schematic created using CircuitLab Since you're using a single-supply non-inverting amplifier, the non-inv input of the opamp should be biased to a non-zero voltage –ideally to Vcc/2 as in your schematic so that the amplified signal can swing equally. Now let's take a look at the ...


12

There are two problems with having the -Vs of a 741 connected to ground. First, as others have said, the output cannot go below ground. In fact, with a 741 it can't even get very close to ground, only to within maybe 1V or 1.5V of ground. This is called the output swing and is usually rated with some kind of a load resistor. Take care that the load ...


12

Okay, after a lot more research, I think I've gotten to the bottom of this. Actually I'm certain it's only approaching the bottom, as I've found this topic area quite deep, but I think I've gotten close enough to shed some light. A basic misconception A turning point in my understanding was when I realized that the equation I led off with in the OP: $$ G =...


11

Short: No!!!! Longer: Without a very specific description of a real world requirement this sort of question is useful at best as a thought experiment. Single stage: So - yes - it can be configured, but it would not be useful. Without looking it up - 10^6 gain = 120 dB which would be in excess of the open loop gain. A bad start. With an input ...


11

It could be either when determined by marketing. Two common configurations are shown below. simulate this circuit – Schematic created using CircuitLab Figure 1a: Fixed gain pre-amp with preceding attenuator. 1b: Variable gain pre-amp. 1a is simple but has the disadvantage that the noise from OA1 is constant even with the input turned to zero. 1b has ...


10

As you say, the decibel is a unit of power ratio. \$ G\ [\mathrm{dB}] = 10 \log_{10}\left(\dfrac{P_1}{P_2}\right)\$. When the input and output impedances are equal and then we can express the gain in terms of voltage as \$ G\ [\mathrm{dB}] = 20 \log_{10}\left(\dfrac{V_1}{V_2}\right) \$ I wouldn't call this the "voltage gain in decibels." I'd rather say ...


10

If you ignore the input capacitor and look at the input impedance into your op-amp circuit from the left of the 200k resistor, the input impedance is the 200k resistor. This is because the op-amp is configured as a virtual earth amplifier. In other words there is 200k loading your capacitor and the 3db high pass cut off point is when Xc = 200k ohms (in ...


10

From Texas Instruments "Stability Analysis of Voltage-Feedback Op Amps Including Compensation Techniques" http://www.ti.com/lit/an/sloa020a/sloa020a.pdf A is the open loop gain (the gain of the opamp itself), and \$ \beta\$ is the feedback resistors. If A\$\beta\$ is very large relative to 1, then the closed loop gain approximates as \$\frac{A}{A\beta}\$ ...


9

What you need to stabilize your converter is the control-to-output transfer function: if a stimulus is applied to the duty ratio input, how does it propagate through the converter and create a response on the output. You can obtain this transfer function in various ways, for instance via a small-signal model like the PWM switch model or by using a piece-wise ...


8

I'm going to say something that you might initially think of as totally wrong: If the voltage gain of a circuit is 6dB, the power gain is also 6dB To produce 6dB voltage gain requires a voltage gain of 2 and 20log(2) = 6.02dB To produce 6dB power gain requires a power gain of 4 and 10log(4) = 6.02dB This means you can talk about gain and not worry ...


8

First we must start with some basic assumptions, we assume that the inputs of the circuit are fed from ideal voltage sources and that the op-amp is an ideal op-amp. The latter means that the circuit is linear and we can apply the principle of superposition to calculate the two gains.. For the inverting gain we treat things as if the + input of the circuit (...


8

Many op amp circuits are designed so that they would yield a known finite gain if constructed using ideal components including an infinite-gain op amp. In practice, such circuits will always be constructed with non-ideal components, and their behavior will not quite match what would have resulted from ideal components. Consider a very basic amplifier: ...


8

Why (microphones) preamps designs tend to limit opamp gain to max 60 dB? A good overall picture of the whole range of what microphones and other audio devices produce: - Picture taken from here. As can be seen a studio microphone (depending on type) can produce a range of outputs from -60 dBm (relative to 600 ohm hence 0 dBm = 0.775 volts) to -20 dBm. ...


8

It's not 3dB absolute, it's 3dB down from the peak, or some sort of nominal attenuation. So in your case, where the passband is -3dB, 3dB down is at -6dB. Note that some filters (e.g. Chebychev) have significant passband ripple; if this exceeds 3dB then the "3dB down" figure loses meaning. In that case, or just if it's what matters to the system ...


7

In your specific op-amp circuit, the voltage on junction of Rf and Rin is the same as the voltage on the non-inverting input. This has to be so - it's called a virtual earth. Given that fact, this means that your signal (Vin) sees an input impedance of exactly Rin. It also means that your output (without connecting to anything else) has to drive an output ...


7

With two voltages (input, output) I would expect that it is likely a "voltage follower". More than that, are you sure about the part number? When speaking about classical voltage opamps the real closed-loop gain for the unity gain amplifier (follower) is \$Acl = \dfrac{Ao}{1+Ao}\$ with Ao: Open-loop gain. Fort a typical value \$Ao=10^5\$ (100 dB) we ...


7

60 dB means 1 mV from the mic becomes 1 V out. That's about the maximum you want to amplify a microphone and feed into a "line level" input. Most microphones produce a few mV out for normal sound levels.


7

The formula you quote is not the gain of an opamp. It is the gain of a circuit containing an opamp and several resistors. That formula only holds when the open loop gain of the opamp is much larger than that given by the formula. When that is the case, the actual value of the opamp open loop gain drops out of the equation. The derivation of the formula is ...


7

Let's start with the definition of the op amp: $$e_{out}= A_{OL}(e_+ -e_-)$$ This is true for every op amp. The device is a differential amplifier, with a very high gain. Now, given that the positive terminal is grounded, $$e_+ = 0\\ e_{out}= -A_{OL}e_-$$ Next, we can apply the assumption that the input impedance is infinite, thus \$i_b=0\$. This let's ...


7

Why is there a resistor in the feedback section of this buffer circuit? If the op-amp has significant input bias currents (rather than offset currents) then making \$R_F = R_1||R_2\$ has some DC accuracy benefit. If the op-amp has an unstable unity gain bandwidth, then using \$R_F\$ might just make the difference between the circuit oscillating or not ...


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