Filters are an important part of analog design. Even circuits that you don’t think of as filters are actually filters. For example, a simple amplifier will have a bandwidth and so above its upper 3dB point it is a low pass filter. Whole books have been written on filters and filter design, both analog and […]
basics
Transimpedance amplifier signal-to-noise
The use of opamps as a transimpedance amplifier is well known and a good analysis of the noise behavior of them is in the old Burr Brown Application Bulletin AB-076 from 1994. This is still available from Texas Instruments’ web site as sboa060 – simply search for that on their web site (Texas Instruments acquired Burr […]
What is a cascode amplifier?
I am not sure how “fashionable” discrete transistor design is nowadays, but anyone using discrete transistors, bipolar or otherwise, for high speed designs other than for switching probably knows about cascode designs. The purpose of a cascode amplifier (not to be confused with cascade which is a chain of two or more amplifiers) is to […]
5 ways to generate a sine wave
It is not uncommon to need a sine wave but how do you generate it? The “best” or most appropriate method for a particular application depends on several things such as: frequency, purity required, amplitude, possible synchronization with another frequency, variable frequency and/or amplitude. At lower frequencies a Wien bridge might be considered – for […]
Input bias current cancellation in bipolar op amps
Bipolar operational amplifiers have an essential input bias current requirement. This bias current has to come from somewhere and can be a nuisance in some types of high impedance circuits such as charge amplifiers and transimpedance amplifiers and can result in offset voltages where input resistances aren’t matched. The example shown below has an input […]
Improving transimpedance amplifiers with a bootstrap
Analog bootstrap circuits are traditionally ones where an output is fed back to the input, usually to increase input impedance. This can be to minimize either the resistive or reactive (usually capacitive) components of the input impedance or both. The term is now also used with MOSFET drivers where a capacitor is charged and used […]
Open loop gain: how to measure with SPICE
When designing opamps in analog IC design, it is frequently required to plot the open loop gain/phase. One way of doing that is to use a high feedback resistor and very high capacitance so the feedback network has an extremely low corner frequency: This works fine unless your opamp drive capability is low, in which […]
