![]() With diodes D1 and D2 in place, the voltage swing across Q1 and Q2 is limited to about ☐.8V, well below the base-emitter breakdown voltage. This protection can be as simple as a pair of back-to-back diodes (D1 and D2) across the amplifier inputs, as shown in Figure 2. Fast swing on op amp input causes potentially harmful reverse bias on Q2 With transistors that have a rated BV EBO of 2 to 3 volts, input protection is clearly required.įigure 1. When the generator transitions from –3 V to +3 V, the amplifier input changes very quickly but the output does not, and a 5.3-V reverse bias develops across Q2. For this discussion, it is assumed that the rise and fall times of the pulse generator are much shorter than the propagation delay of the amplifier. The input of the amplifier is connected to a pulse generator with a ☓-V output swing. The inability of the output to track the input means that the base-emitter junctions of the differential pair could be subjected to a potentially harmful reverse bias over-voltage condition. Real (non-ideal) amplifier outputs cannot respond instantaneously to a change at the input. The amplifier is most susceptible to input stage damage when configured as a voltage follower. For reliable operation it is imperative to avoid reverse biasing the differential pair emitter-base junctions. Breakdown voltage is inversely proportional to process speed, so the faster a process is, the lower the breakdown voltage is. In some higher-speed silicon processes, the base-emitter breakdown voltage (BV EBO) can be as small as 2 to 3 volts. Protecting the differential input stage from damage is accomplished by limiting the voltage across the base-emitter junctions. This protects the amplifier, as ESD currents are routed to the power supplies and bypass capacitors, rather than through the sensitive active circuitry.Ībrupt voltage changes across an op amp’s input can reverse bias the input differential pair, leading to latent defects, increased input bias current, and increased offset voltage. These on-chip diodes are connected from the op amp inputs and outputs to the power supply rails. ESD diodes protect the amplifier against static electricity, electrostatic induction, and other ESD events. Common-mode over-voltage protection primarily limits the input voltage to be compatible with the safe operating range of the amplifier. Various forms of input protection can be found in high-speed amplifiers: common-mode over-voltage protection, electrostatic discharge (ESD) protection, and input differential pair protection are some of the common ones. It also presents alternatives and circuit solutions that utilize amplifiers with input protection. This article discusses the need for input protection, its implementation, and its potential drawbacks. In most cases, this protection is transparent to the user, but in some applications it can be the circuit’s Achilles’ heel. Many of today’s high-speed op amps have on-chip input protection.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |