With all the handheld devices at our fingertips these days, from PDA’s and smart phones, to medical devices and test equipment, it’s no surprise we need them to last as long as possible before giving them some juice.
A key concern, however, is the speed of the op amp since low power usually means limited speed, such as bandwidth and slew rate. I say “usually” because there are exceptions, which we’ll talk about here.
So what do you do if your circuit requires a fairly high gain (20dB or so) but your application can’t really afford all the current needed to boost your speed? This can be a dilemma indeed because we tend to forget about something called decompensated op amps. What benefits can we get from these compared to their unity gain stable counterparts? Simple, we get much better speed. That is, a wider bandwidth and much faster slew rate at no penalty to power consumption.
Bandwidth is expressed as BW=gm/2πCc, where gm is the transconductance and Cc the compensation capacitor. From this formula it’s apparent that the bandwidth is inversely proportional to Cc. Holding gm constant and reducing Cc will surely increase the bandwidth. Does it matter what process we’re talking about? Bipolar, JFET, CMOS? No, the gm of each has different characteristics, but the scheme remains the same.
But what about slew rate? Slew rate is the rate of change of the output over time and is written as SR=dV/dt, which can also be written as I/C. Notice here again the inverse proportionality.
In a decompensated amplifier, there’s no need to boost the current to get either a wide bandwidth or a large slew rate and that’s the primary advantage of these amplifiers. Of course there’s a catch. These are not unity gain stable and will require a minimum gain, usually ten, sometimes five.
To better appreciate the benefits, check out this example:
- The OPA344 has a unity gain bandwidth of 1MHz and a quiescent current of 250µA max while its decompensated version, the OPA345, offers a 3MHz bandwidth for the same current.
- Likewise, the LMP7715 offers 17MHz of unity gain bandwidth and a slew rate of about 8V/us for a quiescent current of roughly 1mA. Its decompensated version, the LMP7717, has a bandwidth of 88MHz and a slew rate of 28V/us for a current of….you guessed it, the same 1mA.
The figure below shows the frequency response of the OPA338, a decompensated op amp, compared to the OPA337, its unity gain stable counterpart.
So the next time you’re looking for a wide bandwidth op amp without having to worry too much about power, think of decompensated op amps. They can be very useful in high gain circuits and when used with other precision devices in a composite amplifier fashion.