There is another effect not caused directly by temperature that can also create instability problems in a poorly designed circuit. called the VBE (at constant IC) varies slightly Early effect. It’s with VCE. This effect is caused by a change in the effective width of the transistor base region as VCE changes. It is given approximately by… = - where ~ 10-4 10-5 (5.9) ΔVBE η ΔVCE η – This is often expressed as a increase of collector current with increasing collector voltage when VBE is held constant. It is expressed as… (5.10) where VA is known as the Early voltage and is typically 50 – 500 V. It is shown graphically in Figure 24. A low Early voltage leads to steeper curves as shown in the graph and this translates to lower collector resistance. Thus, the collector of the transistor act as a good current source. This can have a deleterious doesn’t effect on transistors driving moderately high impedances or on current mirror circuits, which discuss later on. we’ll Figure 24 5.9 Temperature Stability and High Gain Signal distortion and variation of gain can be greatly minimized by applying already demonstrated this negative feedback. We’ve with the addition of RE in the circuit of Figure 23. Also, negative feedback almost completely renders the effects of temperature variation inconsequential. However, this benefit came at the expense of high gain. But could we circumvent this cost so that we could have our cake and eat it, too? Unlike with cake, possible with electronic circuits. The trick is to include it’s
There is another effect not caused directly by temperature that can also create instability problems in a poorly designed circuit. It’s called the Early effect. VBE (at constant IC) varies slightly with VCE. This effect is caused by a change in the effective width of the transistor base region as VCE changes. It is given approximately by… ΔVBE = - η ΔVCE where η ~ 10-4 – 10-5 (5.9) This is often expressed as a increase of collector current with increasing collector voltage when VBE is held constant. It is expressed as… (5.10) where VA is known as the Early voltage and is typically 50 – 500 V. It is shown graphically in Figure 24. A low Early voltage leads to steeper curves as shown in the graph and this translates to lower collector resistance. Thus, the collector of the transistor doesn’t act as a good current source. This can have a deleterious effect on transistors driving moderately high impedances or on current mirror circuits, which we’ll discuss later on. Figure 24 5.9 Temperature Stability and High Gain Signal distortion and variation of gain can be greatly minimized by applying negative feedback. We’ve already demonstrated this with the addition of RE in the circuit of Figure 23. Also, negative feedback almost completely renders the effects of temperature variation inconsequential. However, this benefit came at the expense of high gain. But could we circumvent this cost so that we could have our cake and eat it, too? Unlike with cake, it’s possible with electronic circuits. The trick is to include